Fix a bug in the code that attempts to find pylint

The initial bug was simply an uninitialized variable, but then I noticed that the "get_default_pylint_command" code wasn't using the user preferences at all.  So this version checks the preferences first, and then falls back to searching for pylint, and also corrects the uninitialized var.
Merge pull request #46 from Skorobeus/patch-1

Thanks for the fix!