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).

i'm not sure this field matches up well with our other apis. 

our other objects are all moving towards fully defined reference objects, with either referenceSetIds or referenceIds (see reads.avdl for the best idea of this)

the variant object itself should be defining its reference, so its probably not necessary to restate it here. 
if restating is really necessary though, then this should most likely be a referenceId