Will/external submissions app

* Install `submissions` from an external source: https://github.com/edx/edx-submissions
* Move `openassessment` up one directory level and delete the `apps` directory.  This has the pleasant side-effect of fixing test failures that occur when running `./scripts test openassessment.assessment`

When we merge this and release it to edx-platform, we should add `edx-submissions` to the edx-platform requirements.  I've chosen to use PyPi rather than installing the Git repo directly so we'll get errors if the version in platform doesn't match the version in ora2.

@ormsbee @singingwolfboy  
Looks like the suggested way to do this is with [`pkg_resources`](http://pythonhosted.org/setuptools/pkg_resources.html):
```python
import pkg_resources
try:
    pkg_resources.require("ora2")
except pkg_resource.DistributionNotFound:
    print("not installed")
else:
    print("installed")
```

For a bash script, you could do:

```bash
python -c "import pkg_resources; pkg_resources.require('ora2')" 2> /dev/null
ORA2_MISSING=$?
```
If `ora2` is installed the `$ORA2_MISSING` variable will be set to 0. If it is not installed, the `$ORA2_MISSING` variable will be set to 1.