Implement Advertisement Model

[Raymond Wells]

This will consist of implementing the model for accessning Advertisement info via the API. In addition to the usual requests (GET /, GET /{id}), there also exists POST /{id}/like and DELETE /{id}/like to like/remove a like from an ad.

[Brandon Thai]

Most of the Advertisement model are implemented. Will implement more model if needed.

[Raymond Wells]

Tight. I'll implement the API calls for ad creation today.