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
Suggestion: add support for term qualifiers, which are meta-terms describing, for example, the laterality, severity, or temporal pattern of a phenotype.
```
{
  "id": "HP:0002076",
  "label": "Migraine",
  "qualifiers": [
    {
      "id": "HP:0011463",
      "label": "Childhood onset",
      "type": "age_of_onset"
    },
    {
      "id": "HP:0012828",
      "label": "Severe",
      "type": "severity"
    },
    {
      "id": "HP:0011010",
      "label": "Chronic",
      "type": "temporal_pattern"
    }
  ]
}