Merged class Drop into DropController

closes #112.

* Moved functions from Drop.java into DropController.java
* Deleted Drop.java
* Renamed DropTest.java to DropControllerTest.java
Only the send* methods have a generic type argument. This is needed for [https://github.com/Qabel/qabel-core/pull/204/files#diff-65b0777c060971ccd201f1f6e5bed739R161](sendAndForget(T object, Contact contact)) as we need to know about the class type at compile time. all other functions have only generic types to provide a common interface and be more future proof.
The caller doesn't have to do anything special. It works just as before.