Adding better support for LESS

Hi. Super awesome plugin. 

One thing I'd like to do is use the generated LESS classes as mixins so that I could do something like

    .something {
        .icon-something;
    }

rather than having to place the class directly on the element. 

Mostly really small changes. 
- Added a relativeFontPath option so that I could tailor the location of the font files.
- Updated the templates to output different CSS if the stylesheet option is set to less.

That's it.