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).

so if the API backend has to maintain the uniqueness of this field, then `id` becomes an opaque string which is unique within one API backend. (and can have a value like `sfdkluf76fr`) that object should then have some kind of get method so that you can fetch it. and other objects which refer to it should just use `string scoringAlgorithmId`.

if on the other hand, we don't care so much about the uniqueness of the id field and all the semantics around that, then we should probably just inline the object into its one usage so that GAScore looks like:
```
record GAScore {
  string algorithmName
  string algorithmVersion
  ... other fields
}
```