Some new real-time functions

Hi folks,

This is my first pull-request to obspy, so I hope I'm doing things correctly. 

I'm doing some work on real-time signal processing using obspy, and am submitting the following real-time functions (for now)

* offset (applies a simple offset to the data)
* kurtosis (applies a recursive implementation of a kurtosis calculation)

The corresponding tests are in test_signal.py file, and all tests run.

Let me know what you think!
Alessia