Support Syslog logging.

In production mode, we'd like to log to syslog. This patch adds
support for that (via the syslogger gem). Also, we change the logic
that initializes the logger in the Server, so that in some cases we
can silence the logger (by specifying a nil logdev).

TODO: Override the default log behavior for each mode with command
line options.
Add tests for Commander::Server

Unit tests that mock out all other dependencies, only
testing that the server object calls out to the right
object/methods when provided appropriate conditions.

(/me wonders about excessive use of mocking).