Adding Elmah logging project as an alternative logging option

Hi, 

After asking this question on stack overflow http://stackoverflow.com/questions/20618760/how-do-i-make-topshelf-log-exceptions-to-elmah I decided to add elmah as a logging option.  I've tested this locally and it's sending the correct logs to my elmah database when the service is created and when exceptions are thrown.

There's probably more that can be done here if someone only wanted specific logging levels but that's for another day I think.