Add AMD/browserify support

[mwiencek]

This adds support for AMD loaders like requirejs, as well as browserify, so that a shim config isn't needed in either case. This is pretty standard boilerplate but there are examples for most environments at https://github.com/umdjs/umd

I fixed some problems in package.json that prevented the module from working with npm/browserify. The main field (which indicates the entry file when someone does var tablesorter = require('tablesorter')) was referencing a file that doesn't exist, so I changed it to include the non-minified files and have it reference js/jquery.tablesorter.js as main. The non-minified files are more useful for development/debugging, and people using these tools already have their own minification process for deployment.

[Mottie]

Hi @mwiencek!

Awesome! Thanks for the help :)