Fixes Executing Callbacks Multiple Times

Enter and exit callbacks were being executed multiple times during window
resize. I think this is because there is a delay before the class is
actually added to the element and that is what we are using to determine if
the breakpoint matches. Instead, we use breakpoint name as a data attribute
on the element and set it true or false.
Merge pull request #4 from reusables/fix_execute_callbacks_once

Fixes Executing Callbacks Multiple Times