Module test skip checks

When using `obspy-runtests` with all modules some bound-to-fail network dependent modules take ages to conclude (e.g. http://tests.obspy.org/13968/; the bound-to-fail `seishub` tests make up over 50% of test time). For some modules this is caused by doctests that can not be skipped easily without defacing the actual docstrings.

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 (e.g server unreachable, ports blocked, etc.). This basically extends the skipping of tests implemented in c4dfc4442d71feb85d23310f208472dde61bf82f and 09b5198cfd6835d47c4f44928e444fcbbd57c1c2 to even include doctests acquired by `obspy-runtests`.

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)