Document null handling
Get basic null handling working
Handle cases where an unrecognised null parser is given
Merge branch 'develop' into feature/issue-5-filter-nulls-in-graphite-responses
Fix broken tests after merging in #6
Mention where null handling is configured in the docs
Merge branch 'feature/issue-5-filter-nulls-in-graphite-responses' into develop
Merge branch 'develop' into feature/issue-8-documentation-fixes