Print help message if cargo is invoked with no arguments
auto merge of #412 : ScriptDevil/cargo/default-help, r=alexcrichton

Most modern command line utilities print usage/help when invoked without any arguments. 

cargo (prior to this patch) defaults to saying "no such subcommand" when invoked without arguments. 
> $ cargo
> No such subcommand

With this patch, it will instead print the help message
> $ cargo
> Rust's package manager
>
> Usage:
> ...