Extract manual building code

Squashed commit of the following:

commit 24ee467721cbcbe2edd7f8087679fe1b84f9fa6a
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Wed May 28 12:00:30 2014 -0400

    Remove Prawn::Example namespace shim

commit 1fbf5a10f5a227d1bd8b16f142d4e8348c177222
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Wed May 28 11:45:41 2014 -0400

    Minor tweaks to manual

commit 488c129704505cc102733c748e2207065f98ae04
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Tue May 27 12:13:42 2014 -0400

    Use extracted prawn-manual_builder code

    The functionality of the manual should stay the same,
    we now just depend on an external gem rather than bundling
    the manual building code directly with Prawn.

    In theory, this will allow for the manual building code
    to be used by other extensions. In practice, some
    extra effort may be needed before that's practical.