[ALOY-373] Create <Collection> in markup (singleton)

Allow developers to create a singleton of collections in markup. This will make model-view binding even easier and more expressive. It is also even less code developers will need to achieve model-view binding.

<!-- to enforce readability, <Collection> MUST be a child of <Alloy> -->
<Alloy>
    <Collection src="myModel"/>
</Alloy>

where *myModel* refers to an Alloy model definition that you would find in *app/models*. A collection generated this way will be accessible anywhere in the application via the Alloy.Collections namespace. To access the above, you would do:

var singleton = Alloy.Collections.myModel;