Use the standard Django test command instead of using Nose directly.

This allows the use of the DiscoverRunner in Django 1.7, but should still allow
the use of django-nose if it is specified as the TEST_RUNNER in the django
settings
Add an option for default options for the Django test runner

This allows you to add options that will be passed in when you run the selenium
management command.

For example you could set the following to change the default test pattern for
the Django DiscoverRunner:

SELENIUM_TEST_COMMAND_OPTIONS = {'pattern': 'uitest*.py'}
Use the default browser from the Django settings
Merge pull request #2 from azavea/django-1.7

Get the 'selenium' management command to work with the DiscoverRunner