Make massrel-js's src/* properly support being included as a requirejs package

A few things:

* Convert all dependencies to be relative so that `src/massrel.js` can be the entry point for a requirejs package:
```
{ 
  name: 'massrel-js',
  location: 'bower_components/massrel-js/src',
  main: 'massrel' 
}
```
* Move declaring `window.massrel` into the wrapper so that only the standalone `massrel.js` pollutes the global namespace.

@howardr check over this, especially what I'm adjusting with the global/window.massrel stuff? I ran the tests and they seemed to run fine except for sometimes one that was failing before these changes too.