Added a very basic version of obspy.station.

Aims to provide a hierarchical object structure modelled after
StationXML. Will hopefully be able to read and write from StationXML,
SEED, XSEED, and RESP.
Added Operator, Person and PhoneNumber classes.
Added ExternalReference class
Added inventory object.

Also some more work on the station object.
Added StationXML documentation + definition.
Renamed Inventory to SeismicInventory
Added basic tests to obspy.stations
Skeletion of a StationXML reader.
Merge branch 'master' into obspy.station
Added minimal StationXML test file
Started implementing reading and writing StationXML files.
Reading and writing the minimal StationXML works.

Implemented the basic structure for reading and writing to and from
inventory objects.
Fixing inventory test.
Added writing of ObsPy module tags to StationXML.
Added test for reading other module tags.
Added test for reading and writing the full root element.
Added new test file.

This file has one fully fledged station.
Started to implement reading of the full network file.

Also added parts of a new Comment object.
Added Comment class.
Reading and writing a fully filled network works.