Tooltips for status descriptions

Use the [Bootstrap popover
plugin](http://getbootstrap.com/javascript/#popovers) to show
descriptions of the different statuses.

DRY up index.html template by moving legend information to the
YAML front matter and breaking legend markup out into an include
template.

Still todo: Update microcopy for these statuses.

Addresses
https://github.com/openelections/openelections.github.io/issues/20