Module test skip checks

This PR adds a mechanism to check (with one custom defined function call) in `obspy-runtests` on a module basis if all tests of the corresponding module can be executed or if all tests of the corresponding module should be skipped immediately.

This PR also implements this check for `obspy.seishub`, such that all tests will immediately be skipped (even doctests) if the server is not reachable. Until now, an unreachable server resulted in long timeout
wait time due to doctest executing all tests even after first failures (this behavior can only be altered with doctest.FAIL_FAST starting with Python 3.4, unfortunately)