rename prawn_rails_helper to extension

rename filename and constant from
prawn_rails_helper to extension

and namespace it under PrawnRails
so constant name changes from
PrawnRailsHelper to PrawnRails::Extension
add rails helper prawn_document which builds a prawn document

add rails helper prawn_document which builds a prawn document
it uses the default options for
page_layout, page_size and skip_page_creation
to create the pdf document