Improve documentation about differences between ledger 2.6 and ledger 3.0.
GnuCash support removed, environment variables deprecated.
Change format to "%Y-%m-%d", as described in documentation, and as coded
in report.h
Merge pull request #192 from thdox/report

Change format to "%Y-%m-%d"
Merge pull request #191 from thdox/doc_diff_from_previous

Improve documentation about differences between ledger 2.6 and ledger 3.0