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
fix dayplot in case of problems during event download
docs: dont render image plots as pdf to avoid huge files, adapt Makefile
accordingly, sphinx' latexpdf uses pdf files so replace file extensions in
built tex output files before pdflatex, slightly higher dpi for png
images so tutorial pdf still looks good
docs: add downloadable link to Tutorial pdf (has to be built with "make
latexpdf" independently)

build has to be done like this for this to work:
 - make html
 - make latexpdf
 - make html
docs: adding docs-buildbot script
docs: add links to introduction ipy notebooks on tutorial page
Merge pull request #793 from megies/fix_doc_building

Fix doc building
try to fix travis (see https://github.com/travis-ci/travis-ci/issues/2219#issuecomment-41804942)
deb packaging: include python-dev in build-depends
fixes 2038 problem (see #805)
Conflicts:
	obspy/core/tests/test_utcdatetime.py
	obspy/core/utcdatetime.py
extra test case for #805
reuse TIMESTAMP0 constant
changelog
minor docstring fix (see #808)
Ported #807 to releases branch.

This nables reading INFO request responses from the IRIS ringserver.
Changelog
Removed future imports for releases branch.
Don't accept non-keyword arguments.

Trace.spectrogram() does not accept non-keyword arguments.
Merge pull request #816 from bmorg/fix_spectrogram

Minor Stream.spectrogram doc fix