added string data sysex message support

Added support for sysex `STRING_DATA` messages. I also wrote a passing test case, and confirmed it works using a arduino.

SN: Is there any reason why `read-sysex-event` is private? I'd like to extend it (using `defmethod`) and add my own handlers without forking the project. See [src/firmata/core.clj#L181](https://github.com/peterschwarz/clj-firmata/blob/master/src/firmata/core.clj#L181)