Add a command line target which could be used as a memcached interactive tool

[aisk]

We can add a subfolder and marks it as bin in Cargo.toml, so that we can build a executable with cargo build or cargo install. There is an example: https://github.com/shadowsocks/shadowsocks-rust/blob/master/Cargo.toml

So that we can add a target called memcache-cli, for people can install it with cargo install, and use it as a command line tool to operate memcached data.