Fix instance pdf methods at 0

* Add  check for the gamma distribution instance vs. static pdf method
* Fix distribution instance pdf to allow 0.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Add test for hypergeometric PDF

Adding a test that demonstrates how the hypergeometric PDF sometimes
returns NaN when trying to work with large populations and large
samples.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Wrap up 0.0.3
Now working on 0.0.4
Merging in changes from upstream