Nice bootstrap social buttons PR#640

Hi, 
I have prepared nice social buttons for jade & html and under less & sass.

Stylus is not officially supported yet by bootstrap-social, perhaps it will be.
So could this PR be this still accepted, or it must be fully supported for all options including classical css?

Also I have removed redundant navbar item ``Sign up``, only ``Log in`` left there. In the era of social login nobody really wants to sign up, or am I mistaken?

Cheers,
Andrej