J1939 Rework

[dfieschko]

• Full rework of J1939Message
  • Now supports message generation outside of toJ1939Message()
  • Automatically maintains compliance with RP1210 standard for J1939 messages
  • Properties are now accessed directly, just like with DTC and DiagnosticMessage classes
• Reworked J1939.DiagnosticMessage
  • More efficient and reliable
  • Greatly reduce time complexity of processing large numbers of DTCs
• 100% test coverage of J1939.py
  • Every single line of code is tested at least once
  • Helps maintain compliance with RP1210 standard for J1939 messages
  • Fixed a number of bugs uncovered by new tests
• sanitize_msg_param() will now parse None as b''
• Removed toJ1939Name()
• Add cm() function to J1939.DTC (returns Conversion Method bit)

This update will break some functionality for code using the J1939Message class.

closes #46, closes #35