Merge pull request #94 from ttyS15/master

Try to invalidate m2m in more robust way
Updated CHANGELOG
Refactored/optimized m2m invalidation
Upped to 2.1
A PointField with a None value raises a "ValueError: Cannot use object with type NoneType for a geometry lookup parameter" error when trying to invalidate the object.
Added test for #100
Fix m2m invalidation bug happening on add from promary side

Closes #100
Merge branch 'master' of git://github.com/clearcare/django-cacheops
Skip GIS tests for non-gis databases
Fix test pk race
Proper fix for null geometry fields

Initial fix in #100 works only for PointField and ignores invalidation
for it.
Updated CHANGELOG
Fix silly bug in object serialization
Fix newer geos libraries with older Djangos incompatibility
Add a test for #97
Fix #97
Nicer test skips
Update CHANGELOG
Small cleanup
Upped to 2.1.1