Fail on pylint errors

I'm proposing that we add a pylint check to our build process.  This check will cause the build to fail if there's any major errors such as missing imports, syntax problems, other otherwise obvious issues which should or eventually could cause the code the fail.  For example, this would have caught the missing imports that were fixed in #186 and caused the [Travis build to fail](https://travis-ci.org/pyfarm/pyfarm-master/builds/35302039).

Eventually, other portions of the project should get the same checks too.