GemPlugins and classic plugins consistency

Hello,

http://rails.lighthouseapp.com/projects/8994/tickets/515-make-plugin-generator-create-rails-init-rb

This patch is another step in the effort of having single code-base
for GemPlugins and classic plugins. This will ease migration from
plugins to GemPlugins.

There are also few tickets regarding the other side: freezing and
unpacking gems, loading gem specific tasks and so on. I encourage you
to look (and +1 if you like) at my above patch and these following
tickets (ordering is not random):

http://rails.lighthouseapp.com/projects/8994/tickets/122-rails-init-rb-doesn-t-get-called-for-unpacked-gems
http://rails.lighthouseapp.com/projects/8994/tickets/324-gem-dependencies-inconsistently-load-rails-init-rb
http://rails.lighthouseapp.com/projects/8994/tickets/59-when-loading-a-plugin-via-rubygems-rake-tasks-aren-t-included
http://rails.lighthouseapp.com/projects/8994/tickets/441-add-more-support-for-gems-frozen-in-the-vendor-directory

In particular, please check #59 and the comments section - I don't
want to repeat everything, but it basically says:

"It makes sense that rake tasks loading (and generators) follow the
same logic as init.rb."

Cheers,

Jacek