Added compability for Font-Awesome 4

Font awesome 4 introduced a new naming convention.
I did'nt use new naming convention because:
 * It's backward incompatible and people still use old version of Font-Awesome
 * rails_admin use old naming convention(icon-*) in javascripts.

I added a new scss file which add compability for Font-Awsome 4 uwing extend.
Merge pull request #1814 from bukhamseen/font-awesome-4

Added compability for Font-Awesome 4