'createadmin' command that enforces minimum password length

Partially solves Issue #248. This creates a manage.py command which is almost an exact clone of 'createsuperuser', but enforces the MIN_PASSWORD_LENGTH settings, making sure that the given password should be greater than or equal to said setting.

Also changed all references of 'createsuperuser' in *.py code to 'createadmin'.

@jamalex can you check this out?