Consider renaming crate and repo

[panicbit]

This crate is supposed to be a wrapper around ffmpeg and does not aim at providing compatibility with its fork libav. Naming the crate av or libav could lead to confusing.

Considering that there is already a crate called ffmpeg on crates.io and the trend of naming C wrappers after the library including the lib prefix, libffmpeg would probably be a good name. The repo would be renamed to rust-libffmpeg accordingly.