(DO NOT MERGE) API Add FolderDropdownField to allow users to select the folder to upload into

ref: PLAT-52

This feature introduces a pair of new fields:
* SelectUploadField, which is a subclass of the UploadField control, but includes an additional selector allowing the user to choose the folder to upload to.
 * FolderDropdownField: Which is a subclass of the TreeDropdownField, but rather than intended as a tool for saving to a database (which it can be used for) it's intended to act as a selector for folder. The selected value of this folder persists between each instance of the selector, across the section, meaning that users are able to use a single upload destination when working on many parts of the website.

To be developed: Integration of this feature into the HTML Editor media uploader, so that users may manage their media uploads when working with the HTMLEditorField.

Currently the SelectUploadField looks like the below.

This feature is pending internal review. **Please do not merge**

![screen shot 2014-07-14 at 11 08 40 am](https://cloud.githubusercontent.com/assets/936064/3566058/ac5195da-0ae2-11e4-8a4d-563d69dd2867.png)