rename the btnBackground mixin to btn-states and refactor
rename the btnBackground mixin to btn-states and refactor

This was done to be consistent with BS3's btn-pseudo-states mixin.

up2vs/todc-bootstrap,todc/todc-bootstrap,mozillo/todc-bootstrap,2947721120/todc-bootstrap,2947721120/googlebt4,ztx1491/todc-bootstrap,todc/todc-bootstrap,todc/todc-bootstrap,aaroncnhk/todc-bootstrap