Add an easy way to run tests

It seems that two tests do not pass.

```
Creating test database for alias 'default'...
FF
======================================================================
FAIL: test_bad_label (instances.tests.SimpleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/james/Sites/shared/pulls/django-subdomain-instances/instances/tests.py", line 56, in test_bad_label
    self.assertRaises(ValidationError, lambda: Instance(label='Spaces are not allowed'))
AssertionError: ValidationError not raised by <lambda>

======================================================================
FAIL: test_instance_lower_casing (instances.tests.SimpleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/james/Sites/shared/pulls/django-subdomain-instances/instances/tests.py", line 53, in test_instance_lower_casing
    self.assertEqual(i.label, 'hello')
AssertionError: 'HELLO' != 'hello'
- HELLO
+ hello


----------------------------------------------------------------------
Ran 2 tests in 0.001s

FAILED (failures=2)
Destroying test database for alias 'default'...
```