Add anchors and conditional docroot management

Anchoring keeps the `wordpress::app` and `wordpress::db` classes from
floating off the dependency graphs and causing unexpected ordering
errors.

Conditional docroot management means that you can create a docroot for
this class without it blowing up at you.
Add permissions warning

This is non-optimal, but is the best we can do at the moment.
Merge pull request #6 from binford2k/master

Add anchors and conditional docroot management