Updated models according to current schema

- Compared models with SQL schema in strongSwan sourcecode
- Made changes to be compatible with that reference schema

The goal is that the schema can be generated by django in the future.
The main advantage of that is that we can generate the schema for
different database backends.
Added django-admin configuration file

It is now possible to view, add and edit data from all tables using the
django admin interface.
Created SWID-XML processing function

This adds `lxml` as a new requirement. In contrast to Python's built-in
XML parsers, it's much faster and also provides SAX like parsers that
don't load the entire tag into memory.

The commit also adds .swidtag files for testing purposes.
Configured urls for swid app
Created regid template
Create first version of SWID inventory view
Added jQuery UI datepicker to base template
Added sessions_for_device ajax endpoint
SWID Inventory view now uses ajax to fetch sessions
Added load spinner to indicate ajax loading
js work in progress
Refactored to objects
Implemented first version of inventory view
Added ajax endpoint tests
Merge branch 'issue128-inventory-view' of https://github.com/tnc-ba/strongTNC into issue128-inventory-view

Conflicts:
	apps/swid/templates/swid/swid_inventory.html
	tncapp/models.py
	tncapp/templates/tncapp/base.html