Fix exception in MPTTAdminForm

`exclude` requires a kwarg to filter on.
testcase for #264
Merge pull request #264 from gaqzi/mpttadminformfix

Fix exception in MPTTAdminForm
Merge branch 'master' of github.com:django-mptt/django-mptt
fix note about python support in upgrade notes, we dont support python 2.5 anymore
add supported python and django versions in the readme
Update models.rst

Updated documentation to reflect method rename from get_root(id) to root_node(id)
Merge pull request #268 from ddrabik/patch-1

Update models.rst
0.6.rc2, since a few bugs were found in 0.6.rc1
use either setuptools or distutils in setup.py
docs tweaks. 0.6.0 yay!