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
Epoch time always gives the false sense of precision, so it's impossible to determine if the value is precise or approximate. ISO has flexibility in the precision, where `2004` is a valid date, as is `2004-11-28T14:04:20.123Z`.

Epoch time is hard to process by humans. ISO is human readable.

Epoch time is hard-coded to UTC, which makes it hard to tell if a person was born on January 5th at 12:00 in London, or on January 6th at 01:00 Christmas Island, or on January 4th at 23:00 in Baker Island. ISO defaults to local time, and accepts any time offset.

Both formats are widely accepted standards, but ISO 8601 (and the RFC 3339 profile) seems to be getting more popular on the web.