Improve how we check options for get_law()
Remove todo note

This already works properly, believe it or not. :)
Add a get_url() method

Retrieves the URL for a given section number. In support of #159.
Add a get_related() method

An interface to Solr's MoreLikeThis functionality, to return laws that
are similar to the present one.
Call get_related() from get_law()

Per #159.