Don't destroy global config object

This will encourage people not to destory their window.ENV object by
reassigning it a completely new value.

The environemnt specific files should load after `environemnt.js` has
loaded. This way the values assigned to `window.ENV` in those
environment specific files can overwrite any defaults set in
`environment.js`

I have also cleaned up the comments somewhat and included a simple
example of setting a config value.