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).

> These are defined in the reference genome (e.g. GRCh38.76 or hg38).

NCBI bundles the reference genome and transcript annotations into releases -- but it's not that way at all for UCSC.  "hg38" is just UCSC's alias for the first release of GRCh38 assembly sequences.  We carry many independent transcript sets -- our own "UCSC Genes" set which is typically updated every year or so, Ensembl, GENCODE, CCDS, Vega, Havana, various algorithms' predictions etc.  Each transcript set is independently versioned and updated.  The same ENST* id may appear in several different sets so I think it's important to specify the transcript set and version.