Add UMD support for AMD, CommonJS, and globals

More on UMD here: https://github.com/umdjs/umd

This allows for NVD3 to work with browserify and require.js
Correct file order of concat file
nv now exports correctly
Resolved banner and footer for handling AMD, CJS, and Namespace.
Merge pull request #1 from DavidSouther/UMD

Resolved banner and footer for handling AMD, CJS, and Namespace.
Merge pull request #556 from joeybaker/UMD

Add UMD support for AMD, CommonJS, and globals