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).

The main idea is that this SHOULD be fuzzy and simple.
{1,2,3,...., 42} is more difficult to interpret than {HIGH, MODERATE, LOW}
I'd rather keep them as enums.

I agree that more sophisticated prioritization schemes will emerge in the future (I certainly hope so), and when  algorithms are mature enough, we can write a "variants_prioritization.avdl"