Added FontAwesome support. Cosmetic changes.

* Added support for the "useFontAwesome" boolean setting.
  The setting is stored in a session variable named
  "reactive-table-use-font-awesome", which is then retrieved by
  the reactiveTable template helper "useFontAwesome".

* 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.
  Unfortunately I could only make useFontAwesome a "global" setting,
  since I couldn't figure out how to access the reactiveTalbe's
  template group property from the helper when it gets called from
  within the {{#each}} block.
  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 the leaderboard example to use FontAwesome.