Allow multiple callbacks to be run when a Signal is received.

As Sentry develops, more and more items possibly need to react to signals. To do things like reload config files without restarting the service, a signal (like SIGHUP) is ideal.  Currently Sentry only allows for one function to be called when a signal is received, but different parts of the system may need to react to that same signal.

This is a small change to the SigUtils to allow for multiple handlers for a single signal.