Watching File Descriptors

`GB_WATCH_CALLBACK`	Watch callback function.
`GB_WATCH_NONE`	Constant to stop watching.
`GB_WATCH_READ`	Constant to watch for reading.
`GB_WATCH_READ_WRITE`	Constant to watch for both reading and writing.
`GB_WATCH_WRITE`	Constant to watch for writing.
`GB.Watch`	Starts or stops watching a file descriptor