miniseed: support for (legacy) SRO encoding

As [pointed out on the users mailing list](http://lists.swapbytes.de/archives/obspy-users/2014-July/001425.html), the legacy SRO encoding is not yet supported in miniseed unpacking. For anybody experiencing this a problem at the moment one workaround for IRIS hosted data is to use the `obspy.iris` client with the timeseries web service to request the data in another format (ascii or sac), save it locally and then read it with obspy, e.g.:
```python
from obspy.iris import Client
client = Client()
tstart = UTCDateTime(1982,01,12,01,44,47)
client.timeseries('AS', 'CTAO', '--', 'LHZ', tstart, tstart+100, output="ascii", filename="data.ascii")
st = read("data.ascii")
```

For developers: The format is mentioned on SEED Manual pg. 123. I am adding a test file in this pull request for anybody that wants to work on this.