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). You guys can merge this if if you're happy. You don't need to create a Podspec for this, as the changes offer no real benefit for Pod users.

Fixes #30.

:beers: