Simplify syslog module usage

[okeuday]

• 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.

[okeuday]

I added another function to expose for external validation. It helped make it a bit simpler. Many of the pull requests seem to be invalid based on the current code. Any feedback?