[BUGFIX beta] Do not use ENV if EmberENV exists.

This allows the use of `ENV` in your application without having Ember
hijack it for its uses.

To avoid using `window.ENV`, but still provide customization to Ember's
boot process you could use something like the following:

```javascript
ENV = 'DO NOT USE ME!!!!';

EmberENV = {DISABLE_RANGE_API: true};
```
[DOC beta] minor doc fix
Merge pull request #4020 from twokul/onload-doc-fix

[DOC beta] Minor doc fix
[DOC beta] Make `Ember.computed.collect` visible in docs
Merge pull request #4021 from pangratz/add_doc/computed_collect

[DOC beta] Make `Ember.computed.collect` visible in docs
Merge pull request #3947 from rjackson/do_not_use_global_env

[BUGFIX beta] Do not use ENV if Ember.ENV exists.