Added demography pages for age, education, and gender

@rocha @lamagnifica @clintonb @shnayder @marcotuts 

* Demographics by age, education, and gender are added as as third nav
* Tooltips for charts and metrics are TBD (AN-3736)


Demographics by Age
![demographics-age](https://cloud.githubusercontent.com/assets/5265058/4653535/cca275aa-54ae-11e4-9b47-bd7fc90cdee3.png)


Demographics by Education
![demographics-edu](https://cloud.githubusercontent.com/assets/5265058/4653533/cc88c204-54ae-11e4-839a-2f61c31d6d9f.png)


Demographics by Gender
![demographics-gender](https://cloud.githubusercontent.com/assets/5265058/4653534/cc9811d2-54ae-11e4-9d31-11d36aa18825.png)
Refactored to `test_course_enrollment_demographics.py`.