Addition of a statistics Measurement class

Hello,

I have created another Measurement called `OneMeasurementStatistics` and would like to share it with others in need of some statistical data output. I have modified the `Measurements` class in the way that I added an enum that specifies the existing Measurement types as they can be named in the workload config file. Pleas let me know if the inclusion of my code would require further work. The new class requires the `apache.commons.math` library, which does the statistical stuff except the 95% confidence interval calculation.

*Commit Msg*
It creates statistically oriented output including the mean, standard
deviation and the 95% confidence interval if more than 30 values are
mesured. To select the output type, a third "measurementtype" option
named "statistics" was added to the possibilities.