Delete hard references

The `deleteAllNodes()` method does not work as advertised if there
are hard references anywhere inside of the repository. If someone
is calling `deleteAllNodes()` it is likely that they are desiring
that all of the nodes be deleted, references and all.

For example, `phpcr phpcr:purge` help states:

> The phpcr:purge command remove all the non-standard nodes from
> the content repository

This was not happening if references existed in the repository.
(Possible this was depending on implementation)
Merge remote-tracking branch 'origin/master' into delete-hard-references

Conflicts:
	src/PHPCR/Util/NodeHelper.php
cleaning up purge code
Merge remote-tracking branch 'origin/master' into delete-hard-references
simplifying purge command now that we have fixed the bug in jackalope (both backends)
Merge pull request #33 from phpcr/delete-hard-references

Delete hard references