PDF Thubnmails

Need to enable PDF Thumbnails.

```
apt-get install libmagickwand-dev
source /var/lib/geonode/bin/activate
pip install Wand
```

ALLOWED_DOCUMENT_TYPES and MAX_DOCUMENT_SIZE

```
vim /var/lib/geonode/lib/python2.7/site-packages/geonode/settings.py
```
