Feature: hyperexponential distribution

Hello,
I've recently added the hyperexponential distribution to Boost.Math/Statistical Distribution (with the precious feedback of John Maddock and Paul Bristow).
Here is the pull request log: https://github.com/boostorg/math/pull/7
The contribution has been already  accepted and currenlty it is in the `develop` branch (there may be some additional refinements, but the class is pretty stable).

I thought that it would be very nice to have in Boost the random variate generator counterpart.

So, here it is :)

If you like this idea, can you consider this pull request?

The development includes the random generator class + doxygen doc and two test units.
Note that I had to write my own test units in a similar fashion to what has be done for the `discrete_distribution`).

Thank you so much.

Best,
  Marco