Import early, make the dependency more obvious

If mock was missing, the import error was hidden and hard to troubleshoot.
test_model - python 2.6 compatibility with sets
tests - fix sys.path for proper imports of working copy

Make sure that all tests run with pynag base from working copy
of the code.
Made single test files executable on their own.
Merge branch 'master' of github.com:pynag/pynag