Fix for odd browserify behaviour

Browserify is currently not compatible with backbone-epoxy as it appears it does not support resolving require(package) calls where package is a variable containing a string versus an actual string. E.g.

```js
var underscore = "underscore";
var _ = require(underscore)
```

Does not work. However:

```js
var _ = require("underscore")
```

works as expected. In the former example, browserify complains it cannot find the package. I believe browserify must be doing some strange parsing of the JS files in order to acertain where require() calls are so it can compile a single file.

The very simple solution is to just use strings (most if not all libraries I've seen do this).