gendocs: add option to choose pdf papersize

The default will be US Letter

[ci skip]
gendocs: make html documentation

[ci skip]
gendocs: use hombrew texinfo on mac if installed

The default texinfo (4.8) on Mac OS X shows odd warnings from a sed call
and generates verbatim html tags in links if the part of the link was
marked with @command{}.
With a current version of texinfo (5.2) these oddities are gone.

[ci skip]
doc: try to use groff if man2html is not found

to generate a html version of the manpage,
throw a fatal error otherwise.
doc: add BUILD_A4_PDF cmake variable

to create a DIN A4 paper version of the ledger pdf.
Merge pull request #281 from afh/pull/gendocs

Improve the documentation tools