Roadmap: Allow clear to be called external to the component

[thekevinbrown]

We have a need to be able to clear the signature from outside of the component. I noticed this was on your roadmap so I figured I'd contribute the code back.

This PR does two things:

1. It exposes a prop that allows you to hide the in-built clear button. This prop defaults to false to preserve backwards compatibility.
2. It exposes a method called clear() which can be called from JS to clear the contents of the signature.

I also removed 3 properties from RNSketchManager.h because they weren't being used.

Let me know if any changes would be helpful here, I'm happy to amend as needed!

[thekevinbrown]

Actually I'm going to move off of the master branch on my side, so I've opened up a new PR with the same changes from a different branch: #15