handle None from model __str__; Fixes #753
fix-#759: with_limit_and_skip for count should default like in pymongo
fix-#787: Fix storing value of precision attribute in DecimalField
Merge pull request #793 from DavidBord/fix-759

fix-#759: with_limit_and_skip for count should default like in pymongo
fix PR #787
Merge branch 'DavidBord-fix-787'
Fix merge with AUTHORS
Merge branch 'a4tunado-753'
Update changelog.rst