use my own ffmpeg crate

This significantly improves safety of the ffmpeg interface. The complex
ABIs aren't accessed directly from Rust. Instead, I have a small C
wrapper which uses the ffmpeg C API and the C headers at compile-time to
determine the proper ABI in the same way any C program using ffmpeg
would, so that the ABI doesn't have to be duplicated in Rust code.
I've tested with ffmpeg 2.x and ffmpeg 3.x; it seems to work properly
with both where before ffmpeg 3.x caused segfaults.

It still depends on ABI compatibility between the compiled and running
versions. C programs need this, too, and normal shared library
versioning practices provide this guarantee. But log both versions on
startup for diagnosing problems with this.

Fixes #7

ffmpeg/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "moonfire-ffmpeg"
+version = "0.0.1"
+authors = ["Scott Lamb <slamb@slamb.org>"]
+readme = "../README.md"
+#links = "ffmpeg"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+libc = "0.2"
+log = { version = "0.3", features = ["release_max_level_info"] }
+
+[build-dependencies]
+gcc = "0.3"
+pkg-config = "0.3"