Fixed the breaking of sequences by binding their starting character to a key. Issue #86

To test the fix try the difference between:

    Mousetrap.bind("a", function() {console.log("a");});
    Mousetrap.bind("a b c", function() {console.log("abc");});

and

    Mousetrap.bind("a b c", function() {console.log("abc");});
    Mousetrap.bind("a", function() {console.log("a");});

The latter example now works with this fix. Before it binding the "a" after the sequence would remove the "a" from the sequence thus making it impossible to then do the sequence.

The whole issues is moot for now however because if there is a sequence the first character cannot fire by itself because we don't know if its a sequence or not so the code currently assumes it is and doesn't fire the individual key.

But... there is a feature request out for that too which I'm tinkering with, so this is a precursor.

Side effect of the fix is that you can't re-bind the same sequence.