First commit of the matchmaker APIs

This is a wip translation of the matchmaker APIs into avro.

Autogenerated docs are here: http://cassiedoll.github.io/matchmaker-staging.html
Though the allelic dosage is used in some contexts (and I'm actually the one who originally put this in the original API), I agree that using a VCF-style indexing into ref/alts might provide necessary flexibility down the line.

Also, Richard, I would actually be very hesitant to require prefix bases. Yes it requires the receiving system to actually have a copy of the reference genome to know the exact bases, but these prefix bases can lead to inconsistencies in the representation of indels, and in practice have to just be removed by database-based annotation tools anyway (e.g. CADD, Exomiser, SilVA, ANNOVAR, etc.).