Add Downloads React component to /Examine

This is a link to download the currently selected genotypes as a VCF.
Rework genotypes.py to use SQLAlchemy everywhere.

It's beautiful.
Move relational_vcfs to new common module
Support downloading VCFs
Rewrite views.py to use tables(), other cleanup
Update dpxdt for better sorting, contig sorting

Also handle travis build issue--we don't want to DL and build pysensembl
Address PR

Clean up code, add docstring, move things around
Move Download button over, update pdiff
Address CR, add test for order, restyle
Merge pull request #384 from hammerlab/issue-350-download-vcf

Can now download VCFs, major code cleaning as well