Refactoring and testing.

[ianjosephwilson]

• always use build_cmd_str
• bug fixes
  • writeMicroseconds
  • port detection (those bugs were introduced by prior changes made by me)
• added more ports to port detection
• use logging instead of print statements
• added sr argument to Arduino constructor for testing
• added testing of Arduino class with serial mock object
• added another interactive test for explicitly connecting to a port
• code cleanup
  • fix white space
  • remove unused attribute

I didn't mean for this all to be one pull request but I guess I have to create branches to have the pull requests isolated. Sorry about that.