Features/1294 rebased sensor tests

This closes off #1294 to add basic tests around the sensor code.

This adds basic roboelectric test coverage to the CellScanner and WifiScanner classes.

The primary goal of this is to get some concrete examples of using roboelectric and custom shadow classes to exercise the stumbler code.

Some preliminary documentation has been added into docs/testing.md, but it's still best to look at the example code to see how to write tests. 

We still need to get robotium tests for things where roboelectric's mocking system isn't sufficient and we need a real Android VM to test on.