Use the rust-bindgen crate properly

Using the changes from @gadomski along with some additional work, to get the
generated bindings working again. Works on macOS and FreeBSD 11. A couple of
hacks are needed for FreeBSD, but nothing too serious.

Changed to use the libc prefix, and changed to use the generated enums.

Fixes #22, #15, and #14

cargo test passes

README.md

@@ -10,10 +10,8 @@ A somewhat safe Rust interface to the [ImageMagick](http://www.imagemagick.org/)
     - [Homebrew](http://brew.sh) and [FreeBSD](https://www.freebsd.org) provide this version
     - Homebrew: `brew install imagemagick@6` followed by `brew link --force imagemagick@6`
     - Linux may require building ImageMagick from source
-* ~~Clang (version 3.5 or higher)~~
-    - ~~Or whatever version is dictated by rust-bindgen~~
-* ~~[rust-bindgen](https://github.com/Yamakaky/rust-bindgen) (version 0.19 or higher)~~
-    - ~~This will be installed automatically if it is missing.~~
+* Clang (version 3.5 or higher)
+    - Or whatever version is dictated by rust-bindgen
 * Must have `pkg-config` in order to link with MagickWand.
 
 See the `docs/Development_Setup.md` file for details particular to each platform.