refactor(activity): Refactor into Fragment

The API for interacting with the Activity remains identical. You can now,
however, use the Fragment directly if you want by implementing the
DirectoryChooserFragment.OnFragmentInteractionListener interface which allows
for greater flexibility, like show it as a Dialog or embedding it into the
settings panel on larger devices.
fix(test): rename tests to match requirements

Also add test case for DirectoryChooserFragment

Had to reduce the target SDK to 18, because Robolectric still doesn't support
19.
Merge pull request #14 from passy/fragments

refactor(activity): Refactor into Fragment