better API for passing document parameters

based on suggestion from @jordanbyron
the options hash can now include a documents hash - these parameters are
passed down to prawn, so any other options avaolable via Prawn can be
used (such as background:, info:, optimize_objects, etc. - see
http://prawn.majesticseacreature.com/docs/0.11.1/Prawn/Document.html#method-c-new
)