Add nginx reverse proxy to apache in Vagrant VM

Accessible at port 9001.
Add gunicorn to Vagrant VM
Automatically start dev server in Vagrant VM
Docs: Explain changes to Vagrant VM
Remove null=True on CharFields and others

Reviewed CharFields, EmailFields, TextFields, etc.

Some fields were left unaltered:
* proxy_class rely on unique=True, and that only works sensibly with
  null=True
* The same is true for Product's UPC
* Because order's Line model UPC field usually gets fed the Product's
  UPC, it's least intrusive to still allow null=True there
Subtle changes to handle '' instead of None

Tests failed on a few occasions, because CharFields were set to None or
expected to be None.
Filter by date range in Dasboard/Reporting, if provided
Fix several issues for Django 1.6
Adjust version dependencies for Django 1.6
Disable template timings toolbar panel
Add migration for NullBoolean field
Filter different reports properly
Offer wizard works with JSON session serializer
Fix ordering of conditional offers

Offers by default are sorted using Meta class ordering. However,
`get_offers` was doing two queries and combining them together.
Effecitvely bypass the ordering.

Test for ordering basket offers was written in the process not being
aware of standard ordering. I left it there as it can be useful test in
future (when somebody removes standard ordering).

Fixes #964
Simplify product url tag

Consistently use product.get_absolute_url for product url, instead of 'category:detail' with kwargs
Fix for oscar_calculate_scores

Use double quotes(") instead of grave accents (`) to avoid DatabaseError while running 'python manage.py oscar_calculate_scores'
Add reordering products inside ranges
Fix PEP8
Clean range ordering

- Remove forgotten print statement
- Rename `RangeOrdering` to more generic `RangeProduct` (later we can
  add another fields that are not ordering specific)
- Use `display_order` rather than priority (we set the order using
  jQuery sortable, not bump priority of couple products)
Fix backwards incompatible changes