Add UMD declaration

In order to require this plugin using Require JS or another AMD system
both this plugin and JQuery itself must be AMD-compatible. This PR adds
the Universal Module Defintion pattern from https://github.com/umdjs/umd
so it works as expected with AMD and with traditional methods of
inclusion.
Merge pull request #202 from fidothe/add-UMD-declaration

Add UMD declaration