CIRCLE / django-taggit

When passing a QuerySet to bulk_lookup_kwargs(), the queryset was
undesirably executed to get a list of IDs. This causes an unnecessary
performance loss.

There is no need to loop over a QuerySet to execute a IN (1, 3, 4) query.
Instead, just allow the ORM and database to execute a IN (SELECT id FROM ..) query.
In a real life app with a large set of objects, the difference in performance is noticeable.

Fixed #58 -- capfirst verbose_name when creating the label for form fields, the same way django's builtin fields do.

When building the docs (using tox) also do a linkcheck build to ensure all links work.  Thanks to Daniel Neuhaeuser for pointing this out.

Added a sphinx build to the tox configuration, and fixed a warning in the docs (thanks to Justin Lilly for the assist).