Add SECRET_KEY to test settings
Add .gitignore
Add (failing) test for reloading settings module

Related: #15
Use an importer to load settings module

Fixes #15

Note that settings modules are no longer created recursively. In other
words, if you specify a SETTINGS_MODULE setting of 'a.b.c', 'a.b' must
exist. I don't think anybody was actually doing this anyway; it was kind
of a pointless "feature."