Tolerate minor float rounding errors in cell width calculations.

This is a squashed commit of the following:

commit 58fa18f61b663b3a22e9d1a26ad6f0c3bdd4d441
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Feb 3 12:05:10 2014 -0500

    Wrap spec text

commit 8565dca682cd937bcbcc39bb86ed0a7a5fdcedba
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Feb 3 11:59:35 2014 -0500

    Extract float precision constant and clarify documentation

commit 54432f93ab94d4dd20d65c9fac2fd52f0cc08c98
Merge: f2b2638 7ea6dad
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Feb 3 11:47:13 2014 -0500

    Merge branch 'master' of github.com:prawnpdf/prawn into column_width_arithmetic_errors

commit f2b2638d3853a0965058396798a164a1ac7d7b26
Author: Hartwig Brandl <mail@hartwigbrandl.at>
Date:   Wed Jan 15 20:51:38 2014 +0100

    allowing for small arithmetic errors when calculating column widths