Correct string representation of long event types.

This is done by:
1. Introducing a `force_one_line` option for `AbstractEventType.__str__()`
2. Calling `self.__str__()` with that option in `AbstractEventType.__repr__()`

So that:
```python
In [1]: from obspy.core.event import WaveformStreamID

In [2]: wid = WaveformStreamID(network_code='BW', station_code='FUR', location_code='', channel_code='EHZ')

In [3]: print wid.__str__()
WaveformStreamID
	  network_code: 'BW'
	  station_code: 'FUR'
	  channel_code: 'EHZ'
	 location_code: ''

In [4]: print wid.__str__(force_one_line=True)
WaveformStreamID(network_code='BW', station_code='FUR', channel_code='EHZ', location_code='')

In [5]: print wid.__repr__()
WaveformStreamID(network_code='BW', station_code='FUR', channel_code='EHZ', location_code='')

In [6]: print str(wid)
WaveformStreamID
	  network_code: 'BW'
	  station_code: 'FUR'
	  channel_code: 'EHZ'
	 location_code: ''

In [7]: print repr(wid)
WaveformStreamID(network_code='BW', station_code='FUR', channel_code='EHZ', location_code='')
```