Add api.content.get_path

[davisagli]

It would be nice to have an API for getting an object's path, so that it isn't necessary to use "/".join(obj.getPhysicalPath())

Nice to have: specify whether to get the path relative to the Zope root or Plone portal.

[mauritsvanrees]

As seen elsewhere, obj.absolute_url_path() may be good here.