Adds glyphicon support

These minor changes add glyphicon support when working with bootstrap etc.
![jquery-contextmenu-glyphicon-support-1](https://cloud.githubusercontent.com/assets/1921250/2886688/298eb578-d4ec-11e3-8192-3c8d0aed5cdc.png)
![jquery-contextmenu-glyphicon-support-2](https://cloud.githubusercontent.com/assets/1921250/2886689/298ed986-d4ec-11e3-98ed-44286f9f8bcf.png)