Version 0.5.2

This minor release fixes an unsupported method that came to being used by default Django admin in latest Django update.
Python and Django versions supported (reminder):

• Version 0.5.x will be the last to support Django 1.3.
• Version 0.5.x will be the last to support Python 2.6. Though it may
  still work for the time being, it will be removed from the tested setups.

Fixes:

• Admin does not break anymore on M2M fields on latest Django versions. — #212.
• Related fields's clear() method now works properly (it used to break on MySQL, and was inefficient on other engines) — #212.