obspy-runtests: check on module basis if tests can be run

this commit adds a mechanism to check 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.

this 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)
seishub: use timeout in all requests
Merge branch 'releases' into seishub_timeout

Conflicts:
	CHANGELOG.txt
Merge branch 'releases' into module_test_skip_checks
Merge pull request #786 from megies/seishub_timeout

Seishub timeout
Py 2.6 workaround
Merge pull request #785 from megies/module_test_skip_checks

Module test skip checks
docs: minor fix to API library index
Merge branch 'releases'

Conflicts:
	misc/docs/source/packages/index.rst
	obspy/seishub/client.py
	obspy/seishub/tests/test_client.py
image tests: upload to imgur if image comparison fails with different
sized images
Merge branch 'master' into interpolating