station: handle float derived types

@krischer, we need to do something about many StationXML types that are floats but have additional attributes associated with them. I can think of two ways:

 - flat approach, add those attributes to the parent.. e.g.:

```python
>>> channel.longitude
12
>>> channel.longitude_datum
"WGS84"
```
this would clutter many station classes and feels really ugly and goes along with rather messy code probably.

 - OO approach:

```python
>>> channel.longitude
<obspy.station.util.Longitude at 0x3965e10>
>>> channel.longitude.value
12
>>> channel.longitude.datum
"WGS84"
```
this would look pretty clean but of course most of the times one is only interested in the value and would have to go one level deeper to get the simple float.

This is just an example for handling Longitude/Latitude with the OO approach, still lacking some documentation. Many other fields have to be handled as well..

What do you think? Other opinions?

(Btw. the xml comparison in the tests won't work for randomly ordered attributes..)