Fixed many Ruby warnings when running prawn under 'ruby -w'

In issue #544 it was mentioned that the Ruby interpreter issues many warnings when running Prawn.

This pull request fixes all warnings in the main Prawn library (not pdf-reader nor ttfunk) that show when running `rake manual`.

This means that all

* places where variables were shadowed,
* places where instance variables are not initialized,
* places with mismatched indentation,
* places where deprecated methods are used and
* places with unused variables

that used to generate a warning are now fixed.

Running the specs under Ruby 2.0.0 produced no problems.