Fixed obspy.seedlink bugs caused by mixed usage of class/instance attributes

This pull request fixes the mixed usage of class and instance variables in some of the objects in ``obspy.seedlink``.

The original implementation can lead to unexcpected behaviour, e.g.:

```pycon
>>> station1 = SLNetStation('', '', None, -1, None)
>>> station2 = SLNetStation('', '', None, -1, None)
>>> station1.appendSelectors('FOO')
1
>>> station2.getSelectors()
['FOO']
```

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This pull request also fixes #516.