Add basic Usage documentation.
Per Rust naming conventions, use CamelCase for types:
https://doc.rust-lang.org/style/style/naming/README.html
The documentation includes a basic example that is tested with `cargo
test`.
Update travis.yml to install pure-ftp server in Travis CI build
Use a BufReader around the TcpStream. Writes are not buffered, which I
think is actually best, because we want requests to be issued
immediately.
This follows the "project layout" recommendatoins of Cargo:
http://doc.crates.io/manifest.html#the-project-layout
It allows you to run the example like this:
cargo run --example connecting
The example is also built during `cargo test`. So if you do a change
that breaks the example, you will notice it during testing.
Daniel García