QuakeML misses I/O for Amplitude(s)

`Amplitude` elements do not get regarded right now..

```python
from obspy import readEvents
event = readEvents("/path/to/qml-example-1.2-RC3.xml")[0]
print event.amplitudes
```
```python
[]
```
although there is one in the file:
```xml
      <amplitude publicID="smi:nz.org.geonet/event/2806038g/amplitude/1">
        <type>A</type>
        <genericAmplitude>
          <value>1.0e-08</value>
        </genericAmplitude>
      </amplitude>
```