Extended ordering methods.

This is a rather large proposal to extend django-ordered-model. I'm not sure if there is any interest in merging this ...

Deprecate old methods.
Introduce:
 * up() ....... replaces move_up()
 * down() ..... replaces move_down()
 * down() ..... replaces move_down()
 * swap() ..... swap position with another object
 * to() ....... move object to arbitrary position
 * above() .... move object above reference object
 * below() .... move object below a reference object
 * top() ...... move object to top of stack
 * bottom() ... move object to bottom of stack
Add tests for all new methods.
Make assertions more specific by including the resulting order value.