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).

gotcha. this should then probably be an array, and look a bit more like our other usages:
```
  /**
    A map of additional annotation information. In JSON, this looks like:
    `info: {key1: value1, key2: value2}`
  */
  array<GAKeyValue> info = [];
```