add a lint like mode, detect class decorators

I use pyqver as part of our CI builds since we have to support 2.4. I added a
-l/--lint option to output in the format:

 [pyqver (master %)]$ ./pyqver2.py --lint --min-version 2.0 pyqver2.py testClassDir.py 
pyqver2.py:3:2.4: collections
pyqver2.py:124:2.2: new-style class
pyqver2.py:166:2.2: new-style class
pyqver2.py:342:2.2: False
pyqver2.py:344:2.2: False
pyqver2.py:355:2.2: True
pyqver2.py:357:2.2: True
pyqver2.py:386:2.2: True
testClassDir.py:14:2.6: class decorator
testClassDir.py:19:2.6: class decorator
testClassDir.py:13:2.4: decorator
testClassDir.py:17:2.4: decorator
testClassDir.py:18:2.4: decorator
testClassDir.py:23:2.4: decorator
testClassDir.py:14:2.2: new-style class
testClassDir.py:19:2.2: new-style class

This way I can get similar output to pep8/pylint/pyflakes etc and hook it into syntastic/flymake. 

I also added some detection for class decorators. 

I can split into smaller pull requests if need be, this is just the state of my branch at the moment.

(sorry, don't use any python3 yet, so haven't changed those as well).