Clean up inverse button, adding button control groups etc

Created by: andriijas

1. Clean up of inverse button, replaced last btn-dark reference with btn-inverse
2. Make sure inverse utilizes the already defined variables in variables.less instead of raw colors
3. Adding examples to button groups of btn-inverse with dropdowns etc
4. Adding inverse inline label label-inverse

@markdotto Im thinking of adding alert-inverse as well but there is no default light gray alert so Im have second thoughts. A darker alert is good for growl like notifications.