Features/provide custom ace function

Hi there,

First of all, thank you for taking the time to write this Angular+Ace integration directive.

I'm currently using it with a popup window in which I inject my own Angular template and scope.
That allows me to have an app that opens popup windows and still has the ability to use 2-way bindings with them.

So here's how I'm doing it:

1. Include ace.js in the popup so that it can inject its own css/js properly
2. Use the uiAce directive to show an ACE editor instance inside an angular template (+scope) that is injected in a popup window
3. Modify the ui-ace.js code to allow using a custom `ace` function instead of the one in `window.ace`.
This is because my ui-ace.js file was loaded from the main app's window (not the popup).
4. When providing the config options to `uiAce`, I provide the `popupWindow.ace` function - which is the `ace` function from the popup's `window` object.

And it seems to work fine. :-)

So if you're interested, here's a pull request to add it to your arsenal.

Cheers,

David