Suggestions for #4 and #2, general code janitoring.

* changed broad "except" clause to more specific "except ImportError"
* general PEP8 reformat
* added "debug" option for logging server responses/errors (#2)
* return None instead of True/False on several errors that may be temporary (#4)
  * **True**: Valid
  * **False**: Invalid
  * **None**: Syntax OK but some network problem occurred