Added CONTRIBUTORS.md file

This file lists all contributors, in a way that is common practice among
open source projects on Github.
Changed policy flag dropdowns to multi-select fields

Mulitple SWID-flags and TPM-attestation-flags can be selected,
saved as list, separated by whitspace (tnc-ba/strongTNC#66).
Removed debug print, added policy type object to avoid magic numbers

tnc-ba/strongTNC#66
Removed blacklist flag from package-view and -model

It is save to remove the column 'blacklist' from the table
'package' in the database (tnc-ba/strongTNC#62).
Deleting policies is working again
Added date parser for sortable tables and default sortorder
Added additional TPM attestation flag

tnc-ba/strongTNC#75
Added django debug toolbar configuration

By setting the corresponding environment variable, the django debug
toolbar is loaded. This is very useful for debugging templates and SQL
queries.

Additionally, two unrelated PEP8 violations have been fixed.
Enabled timezone support in Django

This commit enables USE_TZ in settings.py. Additionally, the pytz
library has been added to requirements.txt.
Epoch fields now return timezone-aware datetimes

This is done by explicitly marking datetimes returned from an
EpochField as UTC dates. In the templates, Django automatically
converts timezone-aware dates to localtime.
Fixed tests by always using timezone-aware datetimes
Fixed old usages of native datetimes
Updated copyright line

The copyright now says "© 2013–2014 HSR University of Applied Sciences
Rapperswil & contributors".

The second year will always be up to date as it's dynamically computed.
Merge 'Integration Request 2'

Closes #5.