assets: less flavor of bootstrap

* Installing less and grunt-contrib-less.

* Adding the filters parameter on the assets.
  `{% css "foo.css", "00-bundle", "less" %}`

* Adding some configuration keys.
  LESS_BIN which must be pointing to lessc.
  CLEANCSS_BIN ditto for CleanCSS
  LESS_RUN_IN_DEBUG (default True) to enable or disable the less
  bundling in DEBUG. ASSETS_DEBUG must be also set to True for
  it to work.

* Renamed base.css to base.less and it includes the less files from
  Bootstrap and Font-Awesome.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>