Can configure page size and layout

- add option to configure prawn-rails. has default config for page_size and page_layout
- added helper `prawn_document` which is used to create the pdf document. was in `PrawnHandler` before
  config used in  `prawn_document`
- `prawn_document` helper calls `pdf.render` automatically. so no need to call in code
- skip_page_creation is no longer true. but can set as true in config. not needed as `prawn_document` helper can have option for page_size and page_layout