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).
Should there be a "Seismic" in this line (R<b>S</b>AM/R<b>S</b>EM)?