derive Debug for distributions

[zackmdavis]

This commit adds derivations of the Debug format trait for our
distributions, as is apparently recommended by the std::fmt
documentation ("fmt::Debug implementations should be implemented for all
public types" —https://doc.rust-lang.org/std/fmt/#fmtdisplay-vs-fmtdebug
).

This is in the matter of #118.

[zackmdavis]

(Travis CI failure is nightly only; one can only imagine these errors will need to be fixed anyway before the upstream changes in the compiler hit stable?)

[alexcrichton]

Closing in favor of #137, but thanks forthe PR!