Two new FDSN tests.

Tests the created URLS. Fixing some minor bugs along the
way.
Adjusting test cases once again...
Custom per-service endpoints can now be specified.

Also added a short test.
changelog
fdsn: add possibility to deactivate single services in client (+test)
Added test for actually downloading things with custom mappings.
PEP8
fdsn tests: only initialize clients once
Merge pull request #754 from krischer/fdsn_enhancements

Feature request: override capabilty for specific FDSN web service locations