ENH: Added Pearson Type III distribution.
TST: Added tests for pearson3 distribution.
MAINT: several changes to stats.pearson3

* Moved PDF equation in log form to _logpdf.
* Changed to boolean mask based fancy indexing.
* Now _pdf just takes the exp() of _logpdf.
MAINT: in stats.pearson3, use gamma distribution in the background.
MAINT: more changes to stats.pearson3

* Nearly all pearson3 functions now use the gamma distribution functions.
* Added a _pearson3_preprocess to transform parameters to be able to use gamma
  functions.
* Corrected kurtosis in pearson3.stats.
MAINT: fix references, PEP8 and other small things in stats.pearson3.
Merge branch 'pull-336-pearson'