Rough sketch of formats doc.
Update formats.md
Merge pull request #205 from ambiata/topic/formats

Rough sketch of formats doc.
Fix spelling in formats.md
fixed the scoverage compilation
Remove unused FileSystem import
Change ValueState to handle any ThriftLike value
Extract common setup for priority/namespaced mappers
Changed okCounter var to val
Added type alias MutableFact
Extract PriorityTagDerializer for reducing prioritized thrift values
Pass State function into PriorityTagDeserializer constructor to ensure correct handling