Pull in the log crate and move all the OSX println statements to be at appropriate
log levels.
This also pulls in `env_logger`, an implementation of a logger, that obeys an
environment variable RUST_LOG. I added some notes to README.md as to how to
use it.
Optimally, we don't need `env_logger` except for tests and the binary, but
we can't eliminate it from the library form until [this PR for cargo completes](https://github.com/rust-lang/cargo/issues/1982), so we might need to refactor it out of main.rs when this becomes a Gecko lib.
But maybe not? Anyway, it's easy to change down the line.
J.C. Jones
Taylor R Campbell
rugk
Greg V
Tim Taubert