Smaller and hopefully more stable FDSN examples.

Currently only for __init__.py.
Documentation changes and smaller doctests.
Documentation cleanup.
Caching of discovered services.

This means that repeated initializtion of a client with the same
base URL is cheap.
Printing debug information for the caching.
Changed service hash.
Caching the webservice versions.

This makes interactive use more convenient as the version is
accessed for various help functions.
Fixed receive buffers shared between SeedLink connections.

This should fix obspy/obspy#561.
Fixed spuriously declared class variables.

The way the trace variable was declared allowed for setting a
class variable, effectively rendering every getTrace() call on
every packet useless.

The blockette class variable was unused entirely.
Fixed spuriously declared class variables.

The way the selectors variable was declared led to the following bug:

>>> station1 = SLNetStation('', '', None, -1, None)
>>> station2 = SLNetStation('', '', None, -1, None)
>>> station1.appendSelectors('FOO')
1
>>> station2.getSelectors()
['FOO']
Changelog for obspy.seedlink bugfixes
Added issue number to test case.
Merge pull request #769 from bmorg/fix_seedlink_multithread

Fixed obspy.seedlink bugs caused by mixed usage of class/instance attributes. Fixes #561.
correct some minor flake8 accidentally introduced by #769
Merge branch 'releases'

Conflicts:
	obspy/seedlink/slpacket.py
py3 build: circumvent non-conforming suffix in library names by numpy distutils
library names avoid hardcoded values now and should be only dependent on
what Python's builtin `sysconfig` says (that's what numpy distutils uses
during building internally, too) see #771
refactor 827ae127f88cc39d1, depending on how we import the extensions
(`ctpyes`/`__import__`) we sometimes have to leave away the extension
suffix (see taup). This should be clearer now. Fixes #771.
Replaced some docstrings.

Smaller and older (more stable) requests in the the doctests.
I also made some changes to the documentation structure.
Much faster tests.

I carefully removed some parts and sped up some others as the tests took
too long. I don't think we lost any test coverage but this makes testing
the FDSN client actually practical.