avoid echoing extra \ on non-Mac platforms

This extra \ (which I think was a typo) was causing the revision.tex file to contain "\\newcommand..." on my Linux box.
Merge pull request #6 from sampsyo/patch-1

avoid echoing extra \ on non-Mac platforms
add a QUIET option.

`make QUIET=1` will silence most pdflatex output.  When trying to build the .aux
file out of the .tex sources, a filter will show error messages (with line
numbers) from the log.
generate revision.tex in a cross-platform way
quick start in readme
fix exit status in QUIET mode

Without the parentheses, this was causing the line to always execute false and
exit with status 1.
in QUIET mode, pipe pdflatex output to /dev/null

This suppresses some annoying messages that otherwise leak through, including
"This is pdfTeX..." and "entering extended mode".
use ERRFILTER everywhere PDFLATEX is run

This is probably overkill for what I'm trying to accomplish, which is
suppressing the "leaked" output from pdflatex. It would suffice to just use ">
/dev/null" in these other invocations because they won't be run if the first
pdflatex invocation encounters an error. It is sufficient to do this, though,
and a little easier to write.
fix error regex

[[:digit:]] as a whole is equivalent to \d, so the + needs to go outside of
the whole entity.