First version of metadata

The 2 new files for metadata are in the wip directory so that we can iterate on them a bit before declaring them ready for integration with the other API objects. 

There are 5 new top level objects:
* [GAIndividual](http://cassiedoll.github.io/metadata.html#/schema/org.ga4gh.GAIndividual)
* [GASample](http://cassiedoll.github.io/metadata.html#/schema/org.ga4gh.GASample)
* [GAExperiment](http://cassiedoll.github.io/metadata.html#/schema/org.ga4gh.GAExperiment) (_note this will replace the GAExperiment defined in reads.avdl_)
* [GAAnalysis](http://cassiedoll.github.io/metadata.html#/schema/org.ga4gh.GAAnalysis)
* [GAIndividualGroup](http://cassiedoll.github.io/metadata.html#/schema/org.ga4gh.GAIndividualGroup)

And a search method for each one. 

This PR is a translation of the work done by the metadata task team into avro. I most likely made mistakes! and some of the fields could definitely use a review of their types. 

Autogenerated docs are here: http://cassiedoll.github.io/metadata.html
I meant all the date fields in this repository (so reads, variants, references, metadata etc)
Thanks!