Support for Dagger 2

* Mortar is now agnostic of the dependency injection library.
* Backported Dagger1 support into a small module: dagger1support.
* Added a module for Dagger2 support: dagger2support.
* mortar-sample still uses Dagger1.
* mortar-helloworld now uses Dagger2.
* Removed support for runtime graph validation with Dagger1.
Merge pull request #115 from square/py/dagger2

Support for Dagger 2