Handle signals and termination of main

[facundominguez]

Sending the SIGTERM or SIGINT signals to a sparkle application sends the signals to the process. Handlers should be installed to terminate the thread running  the main function of Haskell. The main thread in turn cannot die with a Haskell exception, or the RTS will kill the application on the spot without giving spark a chance to cleanup the temporary files it has created.