Add conditional for django 1.5 compatibility.
Add properly db lookup handling for array field.
Add more lookups and transforms.

Based on others done by Marc Tamlyn work on django development branch
with some changes for make them compatible with own array field version.
Add documentation for custom lookups.

Some descriptions are taken from django develoment doc made by
Marc Tamlyn (obviously he has better english than me...)
Renames and improvements on tests.