Can configure page size and layout

changes:
- renamed `PrawnRailsHelper` to `PrawnRails::Extension`. constant is now namespaced under `PrawnRails`
- add default config for `page_layout`, `page_size` and `skip_page_creation`
- added a rails view helper `prawn_document` which uses the configured page size, layout and skip_page_creation

PR is useful because it allows us to set `page_size` and `page_layout` in the rendered pdf

have merged https://github.com/cortiz/prawn-rails/pull/14 as well in this PR
needed for getting a green build