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

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 folders. The selected value of this folder persists between each instance of the selector, within the user session, 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.

**Please do not merge**

This feature is pending internal review, tests, and additional documentation. This pull request is intended to support further 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)

ref: PLAT-52