Improve testing function

[paulmueller]

Be a little bit more elaborate after the

__name__ == "__main__"

line in the testing module.

Create plots:

• add appropriate time scale on log-plot (lag time)
• show residuals compared to numpy.correlate and the expected exponential curve
• compare residuals for different numbers of total input array lengths
• create cross-correlation (forward and backward) plots (issue #3)

[paulmueller]

testing module can now be executed as a script and makes a nice plot - included in documentation. No need for residuals at this point.