add get_content method to BaseAsset to fetch the asset contents

[tapple]

A utility method to download asset content. There's also a v2 api to fetch assets. It also requires 2 requests, but it's a json request first, instead of a redirect. A redirect is less code, and none of the metadata in the assetdelivery api looked terribly useful, so i stuck with v1 api which lacks the metadata

https://assetdelivery.roblox.com/docs/