Install and deploy pathagar book server

Ready to merge.

I made some changes to reference implementation in plugins.d/, please take a look and discuss them:

1) Instead of upstream source code we use a custom devel branch in https://github.com/migonzalvar/pathagar/tree/xsce-devel.

2) Custom settings are managed in `prod_settings.py` using Python inheritance mechanism:

```python
# prod_settings.py
from settings import *

FORCE_SCRIPT_NAME = '/books'
...
```

3) Pathagar is installed in a Python **virtual environment** but inheriting packages from the global site-packages directory. This allows to install specific version for some packages using pip (f.e. Django) but use the system ones (f.e. python-psycopg2) for others.


Know issues
=========

- Tags are not added: as a workaround, Django admin site can be used: http://schoolserver/books/admin

These must be fixed upstream or https://github.com/migonzalvar/pathagar/tree/xsce-devel.