[BDG-FORMATS-27] Add structural variant descriptor.
Merge pull request #27 from fnothaft/add-sv-descriptor

[BDG-FORMATS-27] Add structural variant descriptor.
[FORMATS-29] Re-organize the Feature schema

This is attempting to re-organize the Feature schema, along the
discussions that we (Timothy, Uri, Matt, Frank) have had in email and on
the phone.  The main requirements are:
- less file-format dependence in the field choice ('qValue'-like fields
  could be relegated to the 'attributes' field)
- fewer fields to improve the memory footprint