Add PHP Agent installation & configuration

Borrows heavily from [escapestudios/chef-newrelic](https://github.com/escapestudios/chef-newrelic).

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.