…TTF fonts. Ref #56

…TTF fonts. Ref #56

In method annotations.rb:annotate a reference identifier was used instead
of the actual reference. The links in generated pdf weren't working.

Results in pdf were for example :
/Annots [7]

instead of :
/Annots [7 0 R]

…more flexible callback system. Closes #56.

Added a callback system Prawn::Document::Internals#before_render(&block),
which accepts a Proc to be run after all objects are finalized, but before
anything is rendered to the output stream. This removes the need for the
on_encode Proc argument to Prawn::Reference, so that argument has been
removed.

Since we are no longer storing Procs in references, this closes issue 56.
Added a spec to verify.

…ubclasses. This should fix the issue with using Prawn::Format in document subclasses. Closes #57

… word wraps

…sing bundler