Do not create empty files with SQLite ':memory:' with `django_testdir`

 - Keep ':memory:' for DB_NAME and TEST_DB_NAME with SQLite in-memory DB.
 - Skip `create_empty_production_database` with DB_NAME == ':memory:'.
Merge branch 'fix-sqlite-memory-dbname'

Conflicts:
	tests/conftest.py
tests: use 1.7c2 for Django 1.7
Move django.setup() to right after settings. Refs #119.
This patch is still not working properly with regard to loading settings
from within the test directories.
Remove support for having settings within test directories
Fix for error message for invalid settings
Call django.setup() directly after configuring settings
Added changelog entry for #119, #134