Refs #624: Remove "unset" option for contract type

Branch: 624-remove-choice
Refs #626: Drop Django 1.3 testing, add Django 1.5 testing

Updated tox.ini to not test with Django 1.3 anymore, and to
start testing on 1.5.  Got that working. Tried to run tests
on trunk but that's not working yet. For example, the
'django.contrib.markup' template tag library is gone in 1.6
and we use it.

Branch: django15
Refs #655: Don't create two invoices on double-click

This uses a Django 1.4 feature and so should not be merged until we've
dropped Django 1.3 support.

Branch: bug/655-double-click-invoices
Merge pull request #626 from caktus/django15

Drop Django 1.3 support
Merge pull request #624 from caktus/624-remove-choice

Post-585 remove choice of unset contract type
Updated views/URLs with Django 1.4+ conventions
Updated templates & tags with Django 1.4+ conventions
Updated 0.8.3 release date
Updated release notes with unreleased 0.9.0 changes
Remove Django 1.3-related try/except ImportError
Removed backported timezone.py
refs #644 - Add group by year to hourly report
refs #657 - Organize "By Project" on hourly report by project type
OrderedDict isn't compatible in Python 2.6
Moved reports to separate app
Should only require adding 'timepiece.reports' to INSTALLED_APPS.
Migrate reports-specific utilities
Migrate reports templates
Clean up some imports and form names
Merge pull request #672 from caktus/reports-app

refs #671 - Pull out reports into a separate app
More import/form tweaks
@login_required wasn't necessary because having a permission implies being logged in.