[BDG-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
Merge pull request #30 from tdanford/revised-feature

[BDG-FORMATS-29] Re-organize the Feature schema