Introducing Deserializers, and improving core `@api_view` decorators

Main changes:

• Implementing the first Deserializerclass
• Assuring support for all python's versions (2.7+, 3+), and django's versions (1.11+).
• Type hinting to improve code's readability.

Core @api_view changes:

• Supporting deserializer_class parameter.
• Supporting deserializer class mapping (custom deserializers for each http method).
• Adding allow_forms parameter to accept forms data or raise error.
• Supporting for @api_view syntax (only @api_view() was supported before)
• Checking view's type to make sure it's either a function or a django.views.View class
• Normalizing requests payload between content_types, so that the output's always in the same format.
• Extracting query params and passing custom parameters for the view (query params, payload, url params, etc.).
• Decorating only class view's relevant methods (http methods) instead of all methods.
• Moving helpers function into separate file.