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'll be happy to add the *_methods.avdl
I just wanted to have some consensus on how the records should look like before writing them.
If we agrees this is good enough to start writing them, I'll go ahead.