Install EASE and requirements

* Add EASE to edx-platform requirements
* Install fisher and scikit-learn (EASE requirements)

I installed this branch on a sandbox and was able to get the EASE test suite to pass.  The only other thing I needed to do was add this line to `edxapp_env`:
```
export NLTK_DATA="/edx/var/ora/nltk_data"
```

@feanil  If possible, I'd like to get this merged soon because it will make deployment and testing easier.