Use a Math::BigFloat() rather than a regular float for Adjustment comparison.

Based on patch review by Loïc Dachary <loic@dachary.org>, we discovered that
this script inconsistently used 0.02 as a float when comparing against
numbers from the Math::BigFloat() package.  While there were no known bugs
related to this (presumably the 0.02 got coerced into a BigFloat (or
vice-versa) and compared properly), this change nevertheless normalizes to
use of a BigFloat for comparison.
Fix typo in error output.

HT Loïc Dachary <loic@dachary.org>, who noticed this.
Include better explanation of what this report is for in the comments.
Comment to expound more on what it means for  <Adjustment> entries to be ignored by these reports.

Loïc Dachary <loic@dachary.org>, during patch review, pondered whether the
ignoring of <Adjustment> by these scripts could ever be used to hide funds,
ala the movies Superman III and Office Space.

After discussion, we both concluded that it would not be possible to hide
funds merely with this report.  Such hiding would have to also dig into the
main Ledger codebase and muck with how it handles auto-generated <Adjustment>
entries.
Merge pull request #159 from bkuhn/non-profit-audit-reports-updates-20130220

A few minor cleanups in non-profit-audit reports
Merge commit 'e9416ca017902610d7646a4b8312b9d886ea9f31' into next