Added a way to retrieve the OS version for an instance.
Added a way of specifying packages to be installed by OS version.
Omit the @os_version and @capistrano ivars when converting an InstanceItem to a hash.
Support fetching the OS version even when Capistrano is unavailable, if we can.
Make sure instance items have a handle to the Capistrano reference immediately after creation.

Previously the association was only made after booting Capistrano.  This meant running rubber:create followed by rubber:bootstrap worked fine, but broke uber tasks such as rubber:create_staging.
Worked around an issue with Ubuntu 14.04 on VMware.
Updated config to work with both Apache 2.2 and 2.4.
Always use the pgdg apt repository since it supports all Ubuntu LTS releases on all supported versions of PostgreSQL.
Updated the graphite template to work in both Ubuntu 12.04 and 14.04.
Make sure we reference the configured graphite directory.
Use more sensible capistrano hooks for graphite config.
Added in examples of other configuration values.
Simplified memoized value evaluation.
Added a :current_instance helper method that can be used during template processing to refer to the instance the template is being processed on.

This simplifies a common code pattern of searching through the instances list by host name.
Renamed :current_instance helper to :rubber_instance for greater consistency.
Added a utility method to simplify getting the current Rubber instance from within a Capistrano task.