added conftest.py with a pytest hook to skip all tests with the mark "online" if the client is offline
removed useless statements
removed outdated comment
moved definition of constant to the top of conftest.py
extended testing section of dev-guide
explained pytest marks, especially the special behaviour of the *online* mark
Fixed typo in dev guide. Thanks for finding it, @Cadair.
Merge pull request #587 from derdon/pytestnethook

added a pytest hook to automatically skip "online" tests if client is offline