Move towards reactive pattern

[Ecchilon]

This is still a work in progress. It contains a few big changes. The largest is the introduction of reactive patterns through the use of RxJava. Using reactive patterns throughout the app should allow much easier refactoring in the future and makes the API easier to work with in general. The second large change is the replacement of the Apache Http client with OkHttp. This was necessary because the Apache client will be deprecated for Android 23, which I want to start using so the future material design refactoring becomes simpler.
The final change introduced here is retrolambda, which allows for lambda usage in Android even though it's running Java 7. The reason this is done because otherwise I'd become suicidal because of all the (anonymous) inner classes.

There are still a few things broken so this is mostly a WIP. Should be completed soon though.

• [x] MenuBuilder has been deprecated. Find a replacement
  - [ ] Logout seems broken Unrelated to this. See #47
• [x] General clean-up of code

[Ecchilon]

http://reactivex.io/documentation/operators/subscribeon.html

[Ecchilon]

:+1: