Add PHP Agent installation & configuration

Borrows heavily from [escapestudios/chef-newrelic](https://github.com/escapestudios/chef-newrelic).

Changes coming in:

* Add recipe & templates for installation & configuration of New Relic's PHP Agent. The `newrelic.ini` is the latest from the `newrelic-php5` package installation, which means it differs from escapestudios/chef-newrelic.
* Update README to clarify newly-added attributes & recipes.
* Add tests for said PHP Agent recipe.
* Update metadata to reflect new dependencies for PHP Agent install, based on the defaults.

I should note, this tests well on every system except CentOS; it fails to stop the `newrelic-daemon` service for some reason (though if you review the log of actions, it claims it was stopped & disabled). I can't find any fault with the recipe, so I welcome input.