refactoring and fixes #710

refactoring code from cells.rb to column_width_calculator, from now on
all complex width related code is in this file.
fixed #710 be eliminating an edge case as reported under issue #710
improved documentation of fix for issue #710
fixes #721 + test case
removing whitespaces for ruby cop
removing unresolved tag / test case issue 721
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.
Merge pull request #723 from prawnpdf/issue_721

fixes #721 + test case
Merge pull request #712 from prawnpdf/issue_710

refactoring and fixes #710
Remove puts line from column_width_calculator.rb