Implement hypergeometric_distribution as an extension.

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

The hypergeometric distribution describes the process of picking n 
marbles from a bag containing N marbles total of which K are red and the 
remainder are black.
The hypergeometric distribution tells you k - the number of red marbles 
in your hand.

This has been in my tree on x_86_64-linux building and testing clean for 
months.

OK?

Note: Boost has this except for some bikeshed of method names and in the 
deeper issue that my result_type is unsigned integral rather than 
floating point (The numbers of marbles are non-negative integers, dammit).

Ed

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