Merge pull request #24 from nrlucaroni/master

Closed form MLE solutions for some of the available distributions
Added tests for 'Tests.MannWhitney', 'Tests.WilcoxonT' and 'Tests.Sign'
Added tests for multiple comparisons P-value adjustments
Cleaned up MLE functions in 'Distributions'

  Because consistency matters ;)
Abstracted RV type in 'ContinuousDistribution' and 'DiscreteDistribution'

  See #26 for details
Cleaned up 'make_sampler' in 'Distributions'
Added basic tests for summary statistics
Merged 'Mean' and 'Variance' into a single signature 'Features'

  See #26 for motivation
A more efficient implementation of 'make-sampler'
Added skewness and kurtosis to 'Features'
Fixed a typo in 'Base.sample'
Updated to 'gsl-1.13.0'
Cleaned up 'Internal'
Cleaned up and documented 'Distributions'
Added 'LogNormal' distribution
Added 'Bernoulli' distribution
Added 'Logistic' distribution
Added tests for 'Sample.Quantile'
Added tests for 'Sample.rank'
Added missing 'Common' module