Add signal to support additional foreign keys on Sample

Signed-off-by: Adam Wenocur <wenocur@email.chop.edu>
Merge pull request #14 from awenocur/add-signal

Add signal to support additional foreign keys on Sample
added data migration for first phase of enhancing Varify's comprehension of alleles
updated model in migration to reflect changes in issue-2 and variant set merge
first few lines of data migration: still incomplete