Add Browserify / CommonJS support

This makes it possible to require Snap.svg with [Browserify](http://browserify.org/) and modifies `package.json` to point towards the correct module entry point instead of the Gruntfile.

Supporting AMD is great and all, but it's only a few more lines to support CommonJS style module systems when you're already using the factory pattern. Plus, npm is already being used to install `eve`, so I didn't even have to modify dependencies to get things working.

Thanks in advance!