Bundler.autoload(*groups)

Hopefully knocks out #137

Allows you to swap out a `Bundler.require` call with `Bundler.autoload` and things should work (with maybe some Gemfile tweaking)

Added a gem option of `:autoload => 'Symbol' || [:Symbols]` to specify which symbols should cause that gem to autoload.  `:autoload => false` forces the gem to be _required_ and `:require => false` keeps the gem from being autoloaded or required.

The one caveat is that if someone else overrides `Object.const_missing` and forgets to proxy the call back, this facility will break.  (Doesn't seem like a huge concern - most libraries seem to be good about aliasing those calls)

I also have it merged into the [1.0 branch over on my fork](https://github.com/nevir/bundler)