Simplify syslog module usage

* It seems like lager_syslog must require that either:
  1) syslog:start/0 or syslog:start_link/0 was called
  2) syslog_drv.so is in the PATH environment variable for
     erlang:open_port/2 to find as a port driver
  Both of these options have nothing to do currently with starting
  syslog as an application.  This all seems odd, so I just added a
  load/0 function and an unload/0 function to make it easier to use the
  port driver without relying on an extra Erlang process that sits idle
  or the PATH environment variable's extra entry.