Cleaned up @cached_as on several conds patch
Documented @cached_as taking several samples
Clarified that m2m events also invalidate cache
Fixed @cached_as() in python 3
Removed requirement that timeout should not be greater than default

Also removed a possibility invalidator expiring earlier than a key it
references in different model.
Added posarg to tox.ini
Updated CHANGELOG
@cached_as() and @cached() will now depend on function line in a code