Feature: Variant annotations


Hi everyone,
  This is a pull request for adding an Avro IDL file for variant annotations (annotations.avdl). 
  Annotations include records for variant effect, impact, coding and non-coding information, 
clinical relevance, etc.
  It is heavily based on VEP (ENSEMBL's Variant Effect Predictor) and SnpEff (an annotations 
tool I wrote).

btw - we'll need to turn all the one line `//` comments into comments of the form `/** .. */` so that they show up in the docs (the [contributing file](https://github.com/ga4gh/schemas/blob/master/CONTRIBUTING.md#syntax-style-and-conventions) describes this in more detail)