Feature -- Individual Due Date Extension

Implements the individual due date extension feature.  Must be enabled by setting in the environment:

MITX_FEATURES['INDIVIDUAL_DUE_DATES'] = True 

![screenshot from 2013-10-14 11 36 14](https://f.cloud.github.com/assets/148267/1326908/a35e3a0a-34e6-11e3-895e-c71af120faf3.png)