Totally rewrote stacked bar charts

Now a d3.chart component which is totally extensible and has
(mostly) the same functionality as before, plus more.

Added allele frequency calculation and visualization.

Further modularized the project structure.
Merge pull request #5 from ihodes/af_charts

Totally rewrote stacked bar charts
Merge branch 'master' of github.com:danvk/concordance