Added FontAwesome support. Cosmetic changes.

* Added support for the "useFontAwesome" boolean setting.

* Added a missing semicolon in lib/reactive_table.js .

* Changed filter input appearance, from an inline-form to an
  input-group. The left part's content of the input group is the
  localized filer string by default, a fa-filter if useFontAwesome
  is set to true.
  Since by default an input group has width = 100%, I gave it
  col-sm-4 col-sm-offset-8 classes. Floating it right in
  lib/reactive_table.css is therefore no longer needed.

* Changed sorting icons. If useFontAwesome is set to true,
  fa-sort-{desc,asc} will be used.
Updated leaderbard example to use FontAwesome.