function _getReverseMap() {
        if (!_REVERSE_MAP) {
            _REVERSE_MAP = {};
            for (var key in _MAP) {

                // pull out the numeric keypad from here cause keypress should
                // be able to detect the keys from the character
                if (key > 95 && key < 112) {
                    continue;
                }

                if (_MAP.hasOwnProperty(key)) {
                    _REVERSE_MAP[_MAP[key]] = key;
                }
            }
        }
        return _REVERSE_MAP;
    }
reverses the map lookup so that we can look for specific keys
to see what can and can't use keypress

@return {Object}