Update the parse function to accept an entity id

[Rosencrantz]

This includes a tweak to the parse function so that it generates an entity id before creating an entity. There are two ways in which this can occur

1. Supply a list of xpath values that will be concatenated together and hashed in order to generate a unique key
2. Do nothing and allow the parse function to automatically generate a key based on the url of the page that is being parsed.

[sunu]

Could you fix the merge conflict too? I think it's from the changes Simon made to fix a couple of issues in aleph_emit operation.

[Rosencrantz]

This died