Support CommonJS and AMD style requires with phoenix.js

This enables you to do, e.g. `require('phoenix')` when compiling your JS with Browserify and have the Phoenix object be available.

Since the library itself has no dependencies, we don't need to inject anything, but otherwise based on the pattern here: https://github.com/umdjs/umd/blob/master/commonjsStrict.js

I overwrote both the template and `priv/static` versions of the JS file even though your development instructions don't mention the template one, not sure why there are two or what to do there. If that's not right let me know and I'll update the PR.