Improvements Grunt

This library has been really helpful to me. I noticed that the grunt-file could use some improvement. This PR uses the `grunt-umd` module to wrap the distributables in an UMD-module, instead of the current manual method. The footprint of the UMD-module remains exactly the same as before.

I added the newly generated distributables to this PR to show the impact, feel free to remove the commit before merging :)