Improve Gaussian[A] and add Exponential[A].

This commit hooks up the ziggurat algorithm we have for generating
fast normally-distributed Double values, making Gaussian[A] a lot
faster. It also introduces Exponential[A], which also uses the
ziggurat method (and only works for Float/Double currently).
Merge remote-tracking branch 'origin/master' into topic/dist-maker-updates
Merge remote-tracking branch 'origin/master' into topic/dist-maker-updates
Add convenience methods to Uniform, Gaussian, Exponential.

Previously you had to do:

    Uniform[Int].apply(1, 7)

...to dodge Uniform.apply's implicit parameter list.

This commit adds another apply method so you can just say:

    Uniform[Int](1, 7)
Merge pull request #327 from non/topic/dist-maker-updates

Improve Gaussian[A] and add Exponential[A].