Added setup.py to make packages installable

* Created `setup.py` script, which installs packages in this repo.
   - To install locally, use `python setup.py install`.
   - To install using pip, you can specify the GitHub URL in the requirements file.  See [the edx-platform requirements](https://github.com/edx/edx-platform/blob/master/requirements/edx/github.txt) for an example.

* Created an `AUTHORS` file and included it in the installation.

* Added the installation script to the Travis test scripts, to verify that the install process succeeds.

Once packages are installed, you can import them as follows:
```
    >>> import common_grading
    >>> common_grading
    <module 'common_grading' from 'common_grading/__init__.pyc'>
    >>> import peer_grading
    >>> peer_grading
    <module 'peer_grading' from 'peer_grading/__init__.pyc'>
```