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
Implement static_live_server (Django1.7)

fixes #120
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