added feature SSxM

Added obspy.signal.ssxm : method to compute Real-time Amplitude/Energy Measurement and
Spectral Seismic Amplitude/Energy Measurement (known as RSAM, RSEM, SSAM, SSEM).

I tried to be Py3 compatible (only for indexing, I don't know the rest).
> but I prefer numpy imported first, and obspy last

:+1: 
Actually this is even in PEP8: http://legacy.python.org/dev/peps/pep-0008/#imports

```
Imports should be grouped in the following order:

   - standard library imports
   - related third party imports
   - local application/library specific imports

You should put a blank line between each group of imports.
```

Just.. currently this is not handled consistently throughout the code base.