Defining a StdLogger interface to allow setting of any logger

[dcarbone]

We have a use-case where we'd like to be able to specify a custom logger for TrapServer to write to in the event of an error that is not an instance of the built-in log.Logger struct.

There was also a missing %s on line 43 in server.go, not sure if the syntax is OK for you but I've added it anyway.

Comments welcome.

[dcarbone]

It may also be nice to add an ErrorLog field to the ServerArguments struct.

[k-sone]

Thank you for sending a pull request.

I think that the following functions are enough.

	Print(v ...interface{})
	Printf(format string, v ...interface{})

It is because other functions(FatalX and PanicX) are not used.

[k-sone]

There was also a missing %s on line 43 in server.go, not sure if the syntax is OK for you but I've added it anyway.

Thanks! 😄

[dcarbone]

Done and done :)