[RFC] Robolectric module

This provides support for Robolectric tests.

For now, I've added the Robolectric sources to the project because I had to [change the visibility of a few things in RobolectricTestRunner](https://github.com/jacobtabak/burst/commit/dd8c5f14fc27cccc968c86120c99aba876e41fd4) to get this to work.

RobolectricTestRunner was clearly not designed for extensibility, but with a few small changes we could make some things much more manageable (like [`MethodBlock`](https://github.com/jacobtabak/burst/blob/72e7636c03b1f73cde00269b1cd0c1e4c30a151b/burst-robolectric/src/main/java/com/squareup/burst/BurstRobolectricRunner.java#L102-L190) which is mostly copied line-for-line from RobolectricTestRunner).  

So please, share your comments!This provides support for Robolectric tests.

For now, I've added the Robolectric sources to the project because I had to [change the visibility of a few things in RobolectricTestRunner](https://github.com/jacobtabak/burst/commit/dd8c5f14fc27cccc968c86120c99aba876e41fd4) to get this to work.

RobolectricTestRunner was clearly not designed for extensibility, but with a few small changes we could make some things much more manageable (like [`MethodBlock`](https://github.com/jacobtabak/burst/blob/72e7636c03b1f73cde00269b1cd0c1e4c30a151b/burst-robolectric/src/main/java/com/squareup/burst/BurstRobolectricRunner.java#L102-L190) which is mostly copied line-for-line from RobolectricTestRunner).