Refactor http client to its own class, add full API endpoints for feeds

This PR mainly does 2 parts:

* Refactor http part to its own class with helpers
* Add full API endpoints for feed

Note that for each API, we support running in 2 modes: if a callback object is provided, the request will be run in async mode. Otherwise it will run in sync mode.

This architecture resembles other M2X libraries(Ruby, Node.js, etc...) Let me know if something doesn't make sense for Squirrel!

Ready for review.