add an option to provide a SignaturePad object

[marcin-wosinek]

To allow easier integration with projects using webpack; so the following code works:

require('angular-signature');

angular.module('ownModule'), [
  'signature'
].value('SignaturePad', require('signature_pad'));

without putting anything on the global scope.

adding:

angular.module('signature').value('SignaturePad', SignaturePad)

in signature.js & using DI for SignaturePad should do a trick