Overlay decorations

It works:
![overlay-decoration](https://cloud.githubusercontent.com/assets/69169/4987275/9f4b251c-6948-11e4-9d7f-620eb073aa47.gif)

I want to position the `atom-overlay` element to fit within the editor window, so people dont have to think about it. @nathansobo, objections to that?

Closes #3747

### TODO

* [x] Overlay decorations display
* [x] Specs
* [x] Properly position up against the bounds of the window
* [x] Make work when shadow dom is disabled
On autocomplete, when the editor is short/narrow, we end up applying this adjustment and making the situation worse. It may be worth checking that we aren't going negative with the adjustment, that is, that the editor is large enough for the adjustment to actually help.