Fix population tabs in measure view if no population titles were specified

#### Story ####
- https://www.pivotaltracker.com/story/show/67617960

#### Notes ####
- show population <code>sub_id</code> as tab titles if no population <code>title</code> attributes are present

#### Screenshots ####
- ![screen shot 2014-03-17 at 3 54 21 pm](https://f.cloud.github.com/assets/456952/2440932/fb694e2e-ae0d-11e3-87a9-2d4b6b3da91b.png)
- ![screen shot 2014-03-17 at 3 54 27 pm](https://f.cloud.github.com/assets/456952/2440931/fb691e72-ae0d-11e3-942f-d828fa1ac2d3.png)