set default configuration

set default confifuration for
page_size, page_layout and skip_page_creation
PrawnRails::Document has extensions not Prawn::Document

fixed a failing test

also seems a little cleaner that the custom class
has the extension not the base class
add development dependency for pdf-reader

fix failing test
Merge branch 'issue-13-add-development-dependency-for-pdf-reader' into can-configure-page-size-and-layout
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
Merge pull request #15 from deepak/can-configure-page-size-and-layout

Can configure page size and layout