Update filter.py

clarifying docs
Update filter.py
modified filter docs
Data used in the moment tensor should be a list.

It currently is a single element. This tests that multiple
elements are supported.
Data used in the moment tensor is now a list.
Event equality now actually checked across all attributes.
Do not create empty objects if not in QuakeML.

Origin objects will no longer contain empty OriginQuality
or OriginUncertainty objects if they are not specified in the
QuakeML file.
New module obspy.ndk.

Read support for the Global CMT catalog NDK format.
New test file with multiple events.

Edited to test a variety of settings so it does not depict a real
event.
Added test for reading a file with multiple events.
Added test for is_ndk().
Implemented is_ndk() function.
Registering plug-in.
Adding module level documentation for obspy.ndk.
Added test for files with faulty timestamps.
Implemented handling of invalid timestamps.
More error prone.

It can now deal with malformed lines and dates and is just a
little bit more robust. As a consequence it is now able to read
the whole GCMT catalog.
Added large example.

This also shows off the plotting capabilities of the catalog class.
PEP8
Added some notes on the performed conversions.