Added FBSnapshotCase based view test.

This commit is a *reasonably* comprehensive set of initial view tests.
They are fairly fragile, but they provide a good starting block for a
more robust set of tests.

Now we have some tests, we can perhaps be bit more confident in making
changes to this class.

**What's next?**
Next up, possibly after I take a sanity break, will be to add some tests
to ensure that default behaviour never changes between revisions. This
will just ensure if you create a HTPressableButton based on defaults
using this version, it wont change XX versions down the line.

Thanks to @tandavas and @herinkc for their patience in the last [pull
request](https://github.com/herinkc/HTPressableButton/pull/31).

Fixes #30.