Add gem2 installer to use `gem2.0`.

[weakish]

Same as gem installer except it uses gem2.0.

[joshgoebel]

This should be handled by gem supporting a :binary_name option that changes the default from gem to gem2.0 and then someone who needs this can define a helper gem2 function that just calls gem but merges in the binary_name argument.

In fact I'd be ok with gem2 in the API, but it should be part of gem.rb, not a whole new thing.

[weakish]

it should be part of gem.rb

Merged into gem.rb

This should be handled by gem supporting a :binary_name option that changes the default from gem to gem2.0 and then someone who needs this can define a helper gem2 function

I implemented it as an option gem_command.

[joshgoebel]

That's fine, but I also imagined you'd have a gem2 api call that provided an easy way to call it without having to know about the internals of gem_command.

[joshgoebel]

Something like:

 def gem2(name, options = {}, &block)
    gem(name, options.merge(:gem_command => "gem2.0"), &block)
end