Fixes for Arduino Yun

Hi Swift,

I had two issues with BreakfastSerial and Yun board using StandardFirmata 2.3 (bundled with Arduino IDE 1.5.4):

* initialization was failing with `FirmataNotOnBoardException` due to empty `firmata_version` property. It could be caused by changes in Firmata protocol, because 2.3 only responds with queryFirmware (`0x79`) which sets `firmware` and `firmware_version`. Also it's only done after boot, so I added protocol version query on init.

* setting LED value didn't work without setting port mode to output, so I also added it in `Led` init.

I don't have any board other than Yun, so I'm unable to tell if these changes broke anything on other boards :(