Respect the --daemon option and use syslog for logging

Note this fixes [xapi-project/xapi-storage-script#2] but causes
[xapi-project/xapi-storage-script#3] (synchronous syslogging)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #4 from djs55/daemonize

Respect the --daemon option and use syslog for logging