Add ordered class Meta option

If ordered=False, dump returns a regular dictionary instead
of an OrderedDict, which can lead to a significant performance
boost
Field output ordering is False by default

Using normal dicts rather than OrderedDicts leads to a
significant performance improvement
Don't order deserialization output if ordered is False
Add test cases for ordered deserialization output
Merge branch 'feat/unordered' into dev
Update docs: ordering output, changelog