bootstrap: Upgrade from v2.2.2 to v2.3.1.

bootstrap-better-typeahead plugin, needed to allow minLength of zero,
requires newer version of bootstrap than what we currently use.

warn: The base font size increases to 14px.
Merge pull request #43 from npcode/upgrade-bootstrap-v2.3.1

bootstrap: Upgrade from v2.2.2 to v2.3.1.