New API for subcommands

[White-Oak]

As a need for subcommands to be run with vecs appeared(#23), I designed a pair of new traits:

• AmethystArgs is a common trait for Clap-like arguments.
  It has two methods (value_of, is_present) and two implementations (for clap::ArgMatches and Vec<&str>). It is a common denominator to be used in Amethyst CLI and allows to abstractize from the details of implementation.
• AmethystCmd is a trait that should be implemented in order to call something a subcommand.
  It has one method (fn execute<I: AmethystArgs>(matches: &I) -> cargo::CmdResult) which is an entry point to every subcommand. Thus, all subcommands are called via subcmnd_name::Cmd::execute(args).

This design, with little changes to existing code, allows to call executes both with vecs and clap-args.

PS: All subcommands are update to be used with AmethystCmd

[ebkalderon]

Thanks, this looks pretty good. I'll merge it.

[ebkalderon]

Thanks, this looks pretty good. I'll merge it.