Dynamically generate the new_resource attributes

Issue #99 came up because an attribute for the `yum_globalconfig` was left out
- the cachedir. This commit updates the recipe to dynamically walk the
`node['yum']['main']` attributes and send each one with its value to the new
resource. This assures that all attributes defined are passed into the
configuration.

This *can* lead to an error if a user adds an attribute incorrectly. That
seems unlikely, though. I hope.