Change to structured settings files

This changes the formhub/settings.py file to use the formhub/settings directory with multiple settings files.
  
This allows a user to select subtly different settings environments from the command line (or a virtualenvironment) without editing source code.  

For example, two different developers can each use a local sqlite database, while a supervisor test using a MySQL database, while the system runs in production using PostgreSQL -- all using the same settings environment.

Working settings files for Sqlite, MySQL, and PostgreSQL are included.

The changes for pull request 1228 are also included, but formatted differently.

There are no separate tests for this pull request -- it is tested by running "./manage.py test settings=xxxx" with different settings files and observing that the appropriate databases are in fact the ones used.