Allow disabling DEBUG in development

It's really useful to be able to turn off to mirror production
behaviour, ie

```
DEBUG=false python manage.py runserver
```
Merge pull request #22 from sealevelresearch/allow-disabling-debug-mode

Allow disabling DEBUG in development
Refactor, move URL path into top of test class

1. Better for Do Not Repeat Yourself purposes
2. Will enable some general test cases which work across both tide levels &
   windows views (ie testing that the location is validated nicely)
Add 2 test mixins for location & time parsing

Now these can be inherited by other view TestCase classes and they'll
automatically get the same set of tests regarding location, ie

`../tide-levels/<location>/`
`../tide-windows/<location>/`

... will be tested in the same way.

Note that these tests are failing, next commit will fix.