stub out core module

This request is to stub out the core functionality required by an android module which would be shared between the various deployed apps.

At present this set is limited to the wear module (to be completed in another pull), and the app.

The workflow I was thinking was as core functionality is required by features in the wear app, they would be moved from the current ```onebusaway-android``` module into the ```core``` module to keep the core module as lightweight as possible.
I'm leaning towards `org.onebusaway.android.core`, so there is an clearly visible distinction between the core and app classes when working within Android Studio.  Otherwise, I could see someone unknowingly jumping to and modifying a core class, thinking it was only used within the app, and not realizing until later (e.g., Travis CI build) that it actually broke something in one of the other modules.  `org.onebusaway.android.core` would also match the `applicationId`.  Thoughts?