Add correct CSS for date picker

It was using a incomplete bootstrap-datepicker.css. I couldn't find a
better suited replacement, so just switched to an okay looking JQuery UI
theme.

Also switched to the minified version of JQuery UI.
Upgrade timepicker to current version

As part of reviewing datetime pickers, it seemed sensible to bump to the
current version.
Reduce number of evaluated queries.

* Replace "foo.count() == 0" with "not foo.exists()" in python files
* Replace "foo.count() > 0" with "foo.exists()" in python files
* Replace "{% if foo.count %}{% for bar in foo %}" with
  "{% if foo %}{% for bar in foo %}"
* Use 'with' template tag in templates
* Add myself to AUTHORS list
Reduce number of evaluated queries.

* Properly close with tag.
* Properly check for band existance
Reduce number of evaluated queries.

Use 'for..empty' templatetag
Don't use |length filter if not reqired
Serve tracker pixel from Tangent's infrastructure

This is to enable some coarse metrics of who is using Oscar.  We just
look at the referer header to get a rough idea of which sites are using
Oscar.

This is an important measure for Tangent, Oscar's sponsor.  It's
documented in the release notes and we'll make sure everyone knows when
Oscar is released.  It's easy to opt out of.

Fixes #656
Adjust migration to handle data changing part specially

I can't recreate the original issue but, according to a comment by
@vdboor, this change should ensure that the date migration does not
conflict with the schema migration.

Going forwards, I will ensure migrations don't mix data and schema
changes but at this point, it seems better to adjust this migration
rather than renumber (which causes problems for anyone who has already
applied this migration).

Fixes #977 (hopefully)
Mark a few forgotten strings translatable

* Postcode validation message
* Basket price change warning
* "No reviews available" in template

Fixes #1050
Fix .travis.yml for Django 1.6

This should make Travis CI work again.
Merge pull request #1056 from piotrjakimiak/reduce_sql_queries

Reduce query count
New lib to get query counts for test suite

nose-django-querycount gives individual and total query counts. This is
helpful to double-check changes that claim to affect query count
(e.g. #1056).
Total query count gives a quick overview, and diff'ing the output for
two different branches works well.
Move timepicker JS load to dashboard's layout.html

@mbertheau correctly suggested as there's dashboard-global
initialisation going on in dashboard.js, loading the timepicker addon
should be global as well. That's more consistent with how we handle
other dashboard plugins as well.
Merge pull request #992 from tangentlabs/issue/992/datetimepicker

Fix datetime picker in dashboard
Remove ancient import

Not sure how this survived this long.
Correct flash message when saving a variant product