Increase test coverage

## Test coverage when this issue was opened

```
Name                                    Stmts   Miss  Cover   Missing
---------------------------------------------------------------------
incuna_auth/__init__                        0      0   100%   
incuna_auth/backends                       16     16     0%   1-21
incuna_auth/forms                           6      6     0%   1-8
incuna_auth/middleware/__init__             4      0   100%   
incuna_auth/middleware/basic_auth          28     21    25%   7-13, 17-24, 44-53
incuna_auth/middleware/login_required      28      6    79%   54, 61-67
incuna_auth/models                          0      0   100%   
incuna_auth/urls                           12     12     0%   1-38
---------------------------------------------------------------------
TOTAL                                      94     61    35% 
```

![http://25.media.tumblr.com/cd4c30468eed57922ebcfebe3ac89b87/tumblr_mtrwuvN0bu1rqp2rqo1_500.gif](http://25.media.tumblr.com/cd4c30468eed57922ebcfebe3ac89b87/tumblr_mtrwuvN0bu1rqp2rqo1_500.gif)

## Target

Everything except:
- Lines 5-7 of backends.py - these lines aren't executed on Django >= 1.5.
- Line 38 of urls.py - this line isn't executed on Django >= 1.6.  Running the tests with Django < 1.6 would give a different exception, as there are other lines not executed in earlier versions of Django.
- Line 66 of login_required.py - trying to execute this line from a test throws errors about MessageMiddleware not being installed, so we can switch the messaging off for testing.

## Test coverage now (25/04/13, commit 2ccbfe6)

```
Name                                    Stmts   Miss  Cover   Missing
---------------------------------------------------------------------
incuna_auth/__init__                        0      0   100%   
incuna_auth/backends                       16      2    88%   5-7
incuna_auth/forms                           6      0   100%   
incuna_auth/middleware/__init__             4      0   100%   
incuna_auth/middleware/basic_auth          31      0   100%   
incuna_auth/middleware/login_required      28      1    96%   66
incuna_auth/models                          0      0   100%   
incuna_auth/urls                           12      1    92%   38
---------------------------------------------------------------------
TOTAL                                      97      4    96%  
```

![http://jesshaines.com/blog/wp-content/uploads/2013/04/Like-A-Boss.gif](http://jesshaines.com/blog/wp-content/uploads/2013/04/Like-A-Boss.gif)