Support pre django1.6 method of test discovery

fixes #45 on github.

b71ea63390aee61e1bb6484cb07baffd663bdea6 started using a test discovery method introduced in 1.6, but the library supports and tests on 1.5.

The method used works with both 1.5 and 1.6 (which introduced backward-incompatible changes). You have a tests/ package with files that begin with test_.