packages:install improvements

* avoid a apt-get update if dependencies aren't actually defined
* allow global disabling with :packages_install variable
* allow disabling on individual servers with :packages_install attribute
* use new sudo helper
* improved logging
Separate mongrel recipes and don't require :web role

* the long-deprecated mongrel/monit stuff is now only used when
  explicitly required with: require 'brightbox/mongrel'

* allow :web role to not be defined (for unicorn-only app node type
  deployments)

* change default passenger restart strategy variable to :soft
Bundler support improvements

* bundle is now used by default, use :bundle_disable to override
* support setting bundler command with :bundle_cmd
* Run bundler with --quiet
* Set the rake command to use bundle exec
db:create and passenger_restart_strategy doc improvements
move a shared method into helpers.rb
Move mongre/passenger callbacks into appropriate file
Use set, not _cset to set rake
pre-release version 2.4.0.pre1
Added a README