ubuntu: fix unchecked system() return value

gcc 4.8.2 on ubuntu 14.04 says:

flux.c:401:9: error: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Werror=unused-result]
         (void)system (cmd);
         ^
doc: man page typos, etc.
Merge pull request #26 from garlick/misc-fixes

couple minor tweaks to documentation and one ubuntu compilation issue
build: check for a2x program

Abort configure if a2x is not found.
Fixes issue #4
Merge pull request #27 from garlick/autoconf-asciidoc

build: check for a2x program
fix spelling error in flux-snoop(1)
Merge pull request #28 from garlick/misc-fixes

fix spelling error in flux-snoop(1)