Add quest schema

[osrsbox]

The quest schema has been discussed in-depth in #115. This PR moves to add the authored and revised JSON quest schema into the master branch to allow others to more easily work towards adding in quest data into the project.

This PR also adds a pytest module named test/test_quests_database.py. The test can be run using: python -m pytest test/test_quests_database.py when in the root directory of the project. The test is disabled by default so that the project Travis build does not fail. To enable the test, temporarily remove or comment the following line: @pytest.mark.skip(reason="Skipped: The quest data is not currently populated.")

A final summary of a semi-populated OSRS quest in JSON format in provided below:

{
    "id": 136,
    "name": "Dragon Slayer 2",
    "type": "quest",
    "members": true,
    "release_date": "2018-01-04",
    "series": [ "Dragonkin" ],
    "developers": [ "Mod Ed" ],
    "start_point": "Speak to Alec Kincade outside the Myths' Guild.",
    "difficulty": "grandmaster",
    "length": "very_long",
    "description": "Some long winded description of the quest.",
    "url_quest_image": "https://oldschool.runescape.wiki/w/File:Dragon Slayer II.png"
    "url_quest_guide_full": "https://oldschool.runescape.wiki/w/Dragon_Slayer_II",
    "url_quest_guide_quick": "https://oldschool.runescape.wiki/w/Dragon_Slayer_II/Quick_guide",
    "required_for_completing": [],
    "quest_trivia": [
        "A log in screen competition was held...",
        "The first player to complete the quest..."
    ]
    "requirements": {
        "quest_points": 200,
        "quests": [
            50,
            126,
            87,
            117,
            71,
            132,
            131
        ],
        "skills": [
            {  
                "skill": "magic",
                "level": 75,
                "boostable": false
            },
            {
                "skill": "smithing",
                "level": 70,
                "boostable": false
            },
            {
                "skill": "mining",
                "level": 68,
                "boostable": false
            }
        ],
        "items_required": [
            {
                "id": 11920,
                "quantity": 1
            },
            {
                "id": 6739,
                "quantity": 1
            },
            {
                "id": 8778,
                "quantity": 8
            }
        ],
        "items_recommended": [
            {
                "id": 12625,
                "quantity": 4
            },
            {
                "id": 12931,
                "quantity": 1
            }
        ],
        "enemies_to_defeat": [
            8059,
            8056,
            8057
        ]
    },
    "rewards": {
        "quest_points": 5,
        "coins": 0,
        "items": [
            {
                "id": 21880,
                "quantity": 15
            },
            {
                "id": 21892,
                "quantity": 1
            }
        ],
        "skills": [
            {  
                "skill": "smithing",
                "xp": 25000
            },
            {
                "skill": "mining",
                "xp": 18000
            },
            {
                "skill": "agility",
                "xp": 15000
            }
        ],
        "misc": [
            "Ability to speak to cats without the Catspeak amulet.",
            "Access to the Myths' Guild.",
            "Lots of other *stuff*"
        ]
    }
}