bootstrap3: fix form_required_class and form_error_class settings

The wrong keys were passed to get_bootstrap_config. Also add
tests.
bootstrap3: cleanup form_error_class

Give it a default so we don't poke twice with error class, so one
can choose to not add a class to the form group in case of error
with just settings.
bootstrap3: make the bound class configurable

Both in settings and as template tag parameter
Merge branch 'fixformgroupclasses' of https://github.com/xrmx/django-bootstrap3 into xrmx-fixformgroupclasses

Conflicts:
	bootstrap3/bootstrap.py
	bootstrap3/renderers.py
	docs/settings.rst
	testsettings.py
Merge pull request #164 from dyve/xrmx-fixformgroupclasses

Pull in changes for success / bound form class handling