Enhancement - Gemfile syntax for pulling multiple gems from a single github user.

Hi

This patch adds some Gemfile syntax to be able to easily pull multiple gems that are hosted on github in a single users account. Rather than having to define gems individually with the :github option I have wrapped that up into a block. So you can now do this:

    github 'datamapper' do
      gem 'dm-core', '~> 1.3.0.pre'
      gem 'dm-types'
    end

All the standard options for git based gem's are suported (like setting branches to install) so this is really just some syntactic sugar.

Feedback welcome.

thanks