Start making constraint checking optional so that Djangae is GAE budget friendly
Emphasise that the constraint checking is enabled by default
Make sure you can disable/enable constraint checking to your hearts content
Link to the section correctly in the README
Fix some formatting
Fix mistake in the docs
Merge pull request #79 from kirberich/master

Fix IterableFields return None instead of empty iterables
Merge branch 'configurable-constraints' of https://github.com/potatolondon/djangae

Conflicts:
	README.md
Fix typo.