add rudimentary ability to fill PDF forms

First, create a PDF template using any tool that supports embedding PDF Form
annotations.

Then, you load the PDF form as a template, and use the fill_form method to fill
in the blanks.

Example usage:

  pdf = Prawn::Document.new(:template => "form.pdf")
  pdf.form_fields    # => ["name", "email"]
  pdf.fill_form "name" => "Joe Smith", "email" => "joe@example.com"
  pdf.render_file("filled-form.pdf")

Caveat: This currently just places text at the coordinates specified in the
form annotation.  It doesn't yet handle other parts of the specification, such
as font or font size.