Avoid a crash in _quote_params for some default values.

Fix #316.
Bump requirements to Django >= 1.4 & Python >= 2.6
Add Python 3 to tox configurations.

Tests are obviously failing at this point.
Remove unused imports.
Remove compatibility code for unsupported version of Django.
Switch to an external version of sqlparse.
Modernize exception catching syntax.
Avoid naked except clauses.
Replace print statement by print function.
Enable unicode_literals in all non-empty modules.
Switch from str & unicode to bytes & text.
Replace basestring by six.string_types.
Update imports for renamed modules.
Stopped using dict.iter*.

Since performance isn't a primary concern, the non-iterable versions
will do just fine on Python 2.
Allow repeated iteration on DebugToolbar.panels.
Ensure _quote_params returns a list.

This fixes a few test failures.
Replace assertEquals by assertEqual.
Update introspection code.
Update usage of the threading module.
Implement method replacement for Python 3.

Python 3 doesn't have unbound methods.