Make GELF::Logger#add compatible with Logger#add

This changes the interface of `GELF::Logger#add` to be the same as Ruby's `Logger#add`.

Before the change the `GELF::Logger` would loose messages if its `#add` method was used like `Logger#add`.

This changes the code to use the new interface and uses the same technique as Ruby's `Logger#add` to check for messages and facilities in the body of `#add`.

For more information: https://github.com/Graylog2/gelf-rb/issues/26

I really hope this doesn't break anything, since I changed assertions in the tests to conform to the Ruby Logger interface. I also hope I didn't make a mistake while doing this. Careful review would be appreciated :)