Port qiime.util.which to bipy.app.util.which

As the author of ```qiime.util.which```, I, Jai Ram Rideout (@jrrideout), give permission to port ```qiime.util.which``` (GPL code) to the bipy project, which is under the BSD. This also includes the unit tests.

Note that I modified the code from what it originally was to support an env_var parameter.

Fixes #45.