Fill enrollment trend with previous day's count.

@rocha @clintonb 

FYI: @lamagnifica 

Gaps:
![gaps](https://cloud.githubusercontent.com/assets/5265058/4809825/ea3029b4-5eae-11e4-8909-c01367f4d055.png)

Filled Gaps:
![filled](https://cloud.githubusercontent.com/assets/5265058/4809826/ea3f7892-5eae-11e4-8b74-98cc346bb814.png)

Minor: This is creating a datapoint, not a trend. `_create_enrollment_datapoint` might be more appropriate.