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

src/main.rs

@@ -33,8 +33,6 @@
 extern crate byteorder;
 extern crate core;
 extern crate docopt;
-#[macro_use] extern crate ffmpeg;
-extern crate ffmpeg_sys;
 extern crate futures;
 extern crate fnv;
 extern crate http_entity;
@@ -47,6 +45,7 @@ extern crate reffers;
 extern crate rusqlite;
 extern crate memmap;
 extern crate mime;
+extern crate moonfire_ffmpeg;
 extern crate mylog;
 extern crate openssl;
 extern crate parking_lot;