Block Upload Queues

- Add new `Fetchable` Block Mixin as a small wrapper around `$.ajax` calls
- Add new `Ajaxable` Block Mixin that gets mixed into `Uploadable` and `Fetchable` modules.
- Ajaxable has reference to the upload queue and provides a simple way of adding and removing these items.
- Return `jqXHR` objects (deffered objects) in the Uploadable and Fetchable modules 
- If a block is Ajaxable, then kill all requests when that block is removed.
- Modify the `Tweet` and `Image` blocks to make use of this new methods