Prawn PDF bug fix & some features

Hi.

I found some bugs when using has_barcode with [Prawn](https://github.com/prawnpdf/prawn) & [Prawn::Rails](https://github.com/Whoops/prawn-rails).

I wrote model code below.

    has_barcode :barcode, 
      :outputter => :pdf, 
      :type => :code_39,
      :value => Proc.new{|p| p.random_string_of_numbers}

or

    has_barcode :barcode, 
      :outputter => :annotate_pdf, 
      :type => :code_39,
      :value => Proc.new{|p| p.random_string_of_numbers}


First, I fixed require bug. ( #de47b49f , #a68637b9 )

Second, I adopted it with annotate_pdf outputter.
I think prawn_rails need to get Prawn::Document instance. This method seems to be good for it.


It's my first time to do "Pull Request", and I'm not good at English. 
Please read my RSpec & business code. :)