Setting to automatically use UUID primary keys.

This commit adds a new setting, DJANGOPG_DEFAULT_UUID_PK,
which can be set to True to cause UUID primary keys to automatically
be added in lieu of the Django default AutoField.
Add support for `select_related` and `prefetch_related` in Meta.

This commit adds support for including select_related and
prefetch_related as Meta options, for convenience.
Documentation for 1.3 features.
Version number bump.
Bumping the next version number in Sphinx.