Teach `twigg app` -D/--daemon and -P/--pidfile options

In order to make the Twigg app easy to run as a service (eg. at boot
time) or monitor (eg. with monit), this commit teaches it some options
for going into the background as a daemon, and writing out a pidfile.

If a pidfile already exists, it bails. Note that we first employ a
race-prone check, before we daemonize, in order to be able to print a
diagnostic warning. After we've daemonized, we check again (trying to
obtain an exclusive lock) and we might die if we lose a race, at which
point it's too late to print out anything diagnostic because our stderr
and stdout have ben redirected to /dev/null.

Change-Id: I721aef639ad53c0584c44666fa11b925b25538bc
Reviewed-on: https://gerrit.causes.com/27578
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Bump version numbers for 0.0.3 release

Note the way this is currently set up it is most convenient if we
bump the version numbers of all the gems in lock-step with one another.
I might loosen this requirement in a future commit, to make it possible
to update just one of the gems and have the others track it via some
sort of "~> X.Y.Z" requirement.

Change-Id: I62647d7797619a1f143c7530bac7d823b7d3aa5e
Reviewed-on: https://gerrit.causes.com/27579
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>