Add UI for downloading various archive formats.

Change the “Download” button to a split button with options for zip and
tar.gz.
Added download option to tree view.

### PR #5891 must be merged first in order for this UI to work properly.

#### New Project Download Buttons
![repo-download](https://f.cloud.github.com/assets/2117996/1834309/d066c4a8-73da-11e3-8800-208c7fc45206.png)

#### Old Project Download Button
![repo-download-old](https://f.cloud.github.com/assets/2117996/1834350/4d308636-73db-11e3-844b-2b28ddf7ab24.png)

#### New Branch/Tag Download Buttons
![branch-download](https://f.cloud.github.com/assets/2117996/1834311/d454e27a-73da-11e3-8a16-c1aa027b8a16.png)
![tag-download](https://f.cloud.github.com/assets/2117996/1834314/d6551b30-73da-11e3-82b7-0ce8becd5a29.png)

#### Old Branch/Tag Download Buttons
![branch-download-old](https://f.cloud.github.com/assets/2117996/1834349/4d29931c-73db-11e3-9e18-ea737464c77a.png)
![tag-download-old](https://f.cloud.github.com/assets/2117996/1834351/4d37d5c6-73db-11e3-99cb-cd9d67ba3683.png)

#### New Tree Download Buttons
![tree-download](https://f.cloud.github.com/assets/2117996/1834315/d8a9479e-73da-11e3-9825-a18630974c51.png)