Replace 'gem verify' and 'gem vinstall' with 'gem install --verify'

[grant-olson]

We should have a less clunky interface. We should deprecate these two commands and incorporate the functionality into the existing install command.

Now that I'm digging into the rubygems code, it seems I can add command line parameters to existing options and hook in via Gem::CommandManager.instance[:install].add_option and hopefully hook into a pre-install hook.

• if you specify --verify, we should either verify the gem or abort with an error.
• The additional option --get-key should try to retrieve the key from the keyservers.
• the existing commands gem verify and gem vinstall should provide a message explaining that syntax is deprecated and show the new syntax.

[grant-olson]

Actually, the vinstall command never got implemented so that doesn't need to go.

And it probably makes sense to keep the stand-alone verify command.