Integrate RoboElectric for testing

[isuPatches]

Generally, I would not be in favor of adding a dependency like RoboElectric given the numerous outstanding issues with the tool, but in the case of this Library, it's benefits will outweigh the cons. My thoughts are:

1. We can move to full unit tests for the library
2. We can emulate multiple Android SDK levels and easily test against an array of them as necessary
3. We can generate a full code coverage report without having to aggregate across unit + instrumentation tests from different devices with different SDK levels
4. Travis CI with emulators absolutely sucks

I foresee this being an interim solution and would consider Firebase TestLab or another similar service as being the ultimate end goal.