BUG/TST : skip example pep8 if don't know source path

closes #3596
TST : made known-failure message more useful
TST : made search for examples smarter

Will find examples if cwd is anywhere is the
source tree.
Merge pull request #3597 from tacaswell/pep8_fix

BUG/TST : skip example pep8 if don't know source path