Code.org Integration

[epeach]

These changes facilitate the integration of the MicroBit Firmata client code into the Code.org repository: https://github.com/code-dot-org/code-dot-org.

This PR contains 3 changes:

• Allow serialport to be overridden in the constructor so that we can provide our own implementation.
• Detect when this code is run in a browser and use the global TextEncoder/Decoder in that case.
• Return a promise from connect and setSerialPort so that client code can determine when those functions are complete.

cc: @joshlory

[JohnVidler]

Sorry this has taken so long to address.

This looks like a reasonable set of changes with minimal impact on our own tests, so lets just get this merged in.