Saving and Loading of compressed PPSD files

I extended the PPSD object's `save()` method to support saving of compressed files. I also added a `PPSD.load()` to seamlessly load pickled PPSD objects, both compressed and uncompressed.

I tested this on Debian and Windows 7.

I also updated the documentation and the unit test.

I did, however, not get the doc builder to run, so I was unable to check if I got the doc syntax right. Especially this reference to the static method:

```
    The :func:`~obspy.signal.spectral_estimation.PPSD.load` method detects
    compression automatically.
```