Correction of the Pk metric

The first implementation I did was 100% wrong.
According to [1] Pk should provide a penalty of about 0.5 to all degenerated segmentations (empty, full, randomized), which is not statisfied by my last implementation.
It was corrected using the explantations provided in [2]

Sorry about that

[1] Beeferman D., Berger A., Lafferty J. (1999)
Statistical Models for Text Segmentation
Machine Learning, 34, 177-210

[2] Unbiased discourse segmentation evaluation
John Niekrasz and Johanna D. Moore
Spoken Language Technology Workshop (SLT), 2010 IEEE
Page(s): 43 - 48
better tests
Merge pull request #353 from DavidDoukhan/master

Correction on the Pk segmentation metric