Improve schema generation on arrayMergeById

Conflicts:
	tests/test_jsonmerge.py
Explanation for $ref workaround in arrayMergeById
Add test where get_schema descends twice into $ref
Merge branch 'fix-arrayMergeById-get_schema'
PEP8 cleanup
Update None is ignore, empty string is nullifying a value
Remove maxDiffs that are hanging around
Don't update on null (not empty string)
Allow versioning on referenced items.
Merge branch 'master' into ocds

Conflicts:
	jsonmerge/strategies.py
	tests/test_jsonmerge.py