Move database connection initialization outside of settings module

Addresses https://github.com/openelections/core/issues/223
Allow settings to be loaded from an environment variable

Instead of always getting settings from ``openelex.settings``,
allow settings to be loaded from any Python file pointed to by
the ``OPENELEX_SETTINGS`` environment variable.

This allows the user's settings to live anywhere on the filesystem
and outside of a directory on the PYTHONPATH.  This should make
deployment easier as well as allow ``openelex-core`` to function
more like a "normal" Python package.

With this change, settings should now be accessed from the
``openelex.config.settings`` variable instead of
``openelex.settings``.

Addresses https://github.com/openelections/core/issues/63
Merge pull request #224 from openelections/63-settings_outside_project

Support settings definition outside of package directory