Let the help command work consistently everywhere

This adds a dummy help command so that it's usage can be documented with docopt!

Also adds help flags to all of the subcommands that were missing them. Without
that `cargo help sub-command` shows Invalid Argument before the usage text.
auto merge of #667 : jakerr/cargo/help-help, r=alexcrichton

This adds a dummy help command so that it's usage can be documented with docopt! This lets `cargo help help` work.

Also adds help flags to all of the subcommands that were missing them. Without
that `cargo help sub-command` shows Invalid Argument before the usage text.