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).

should this object be a number instead of an enum?
like something between 0 and 1? high = 1, modifier = 0, etc

the reason i ask is because this seems a bit fuzzy, and doesn't allow for better estimates over time. if we come up with some amazing algorithm that knows 10 times as much as we do know about impact, it will still be stuck with just 4 enums. 

(plus numbers have nice default sorts :)