Rather than cloning the bindgen repository and building from source, use
the packaged crate. The result is the rebuilds are much, much faster.
Also, had to make numerous changes for the new version of bindgen.
cargo test passes
Leave the generated bindings out of the source repository and instead
have Cargo automatically build them if they are missing. This allows the
version of MagickWand to be incorporated into the bindings
automatically. For instance, in Homebrew the version is 6.Q16, while on
FreeBSD 10.2 it is 6.9, and only be generating the bindings for each
system can we ensure a smooth compilation process.
