Adds DTC support

[iwanders]

This PR adds a huge chunk of work:

• A method to read a variable length response from the ECU
• Method readTroubleCodes() to request trouble codes from the ECU (mode 0x03).
• Method readPendingTroubleCodes() to request pending trouble codes from the ECU (mode 0x07)
• The clearTroubleCodes() method is now tested and confirmed to be working (mode 0x04).
• A method decodeDTC to decode the binary representation of the trouble code into ascii.

Huge thanks to @produmann and @crashmaxx for helping bring these features to the library.

Closes #8, closes #1 and closes #9.