[BDG-FORMATS-31] Added sampleId to Pileup

This fixes the over-aggressive deletion of the sample fields from the
previous revision to the Pileup schema.
Merge pull request #32 from tdanford/pileup-counts

[BDG-FORMATS-31] Added sampleId to Pileup
[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