* Adds .ng_parse_options as accessible attribute to Openvault module's class, and overwrite ng_parse_options= to raise an error if you try to set it to something other than an integer (otherwise you'll get a funky error thrown from inside of Nokogiri somewhere).

* Adds Openvault::XML as a method _and_ a module so you can use in the same was as Nokogiri::XML.. but it uses Openvault.ng_parse_options.

Closes #137.