Explicit EventHandler creation avoiding reflection

The motivation behind this changes is to be able to create and register new event listeners in run-time, instead of depending on annotation reflection.

The original API for Bus instances is intact and all tests are passing.

This changes have been testes in the sample project and in a sandbox project loading Otto as a Maven dependency.

Commit message: 
New register & unregister methods in Bus for explicit (not by reflection) EventHandler definition
Added a generic Callback interface to be extended (implemented) by third parties
Added a specific implementation of MethodCallingCallback with the original implementation of an EventHandler created by reflection of @Subscribe annotations in objects
Added tests to cover added features
Added example of explicit registration&un-registration of Callbacks in sample project