Feature Addition: Ability to have custom mbtile overlays

Hey, 

I wanted the ability to create a "operational" layer in tilemill which I could then overlay with field papers.  The basic flow is the following:

- User must be logged in
- User clicks "Extend" Nav
- User clicks MBTiles Uploader Tool 
- User uploads .mbtiles file to account
- User selects "Make"
- Make Step-1 has 2nd the option to use custom uploaded tiles
![image](https://cloud.githubusercontent.com/assets/433221/5491650/bf64986a-86a8-11e4-8420-6c9dff6d5af1.png)

- Make Step-2 would now have a separate dropdown to control the overlay layer:
![image](https://cloud.githubusercontent.com/assets/433221/5491672/e0e62882-86a8-11e4-84ac-06a3df12ed5f.png)

- Overlay selection would carry through to atlas pdf

I have implemented the bulk of this, but am still testing.

One debatable issue I ran into was the empty tile.  Because the overlay mbtile mostly be empty tiles to let the base layer shine through,  I commented out the empty tile logic:
![image](https://cloud.githubusercontent.com/assets/433221/5491725/6c1ef384-86a9-11e4-818d-866d59fbd600.png)


So this is NOT ready to officially merge to master yet, but is functional and I wanted to get some feedback.

Thanks.