rename `signature` to `ng-model`?

[coolaj86]

It seems that most directives use the de facto standard of calling the main model ng-model.

Perhaps you can release a version that uses ngModel and is always backwards compatible with accepting signature?

Also, I think it would be a good idea to rename signature-pad to mr-signature-pad to avoid name conflict with the other 3 or 4 modules named some variariton of signature_pad, sigpad, signature-pad, etc that are popping up, some of which also have angular directives.