XAuth

* Implemented XAuth.
* Excluding IntelliJ files in `.gitignore`.

For Android developers that want to integrate Tumblr functionality into their app, authentication is a huge barrier. This should make using Jumblr on Android a snap and (hopefully) encourage developers to include Tumblr functionality when making their apps (especially with the new Gradle build system).

Tumblr responds in a different structure for XAuth than it does for all other requests, so it made adding these pieces a bit awkward in the current infrastructure, but I tried to capture the spirit of the current architecture.