Slidy remote: fixed call to removeEventListener for "gesturechange"

The call did not specify the event handler to remove.
Presentation API shim: more logging messages to track potential issues

Logging messages are now disabled by default. Caller needs to set
navigator.presentationLogLevel to one of "log", "info", "warn",
"error", "none" or null (which also means "none") to see messages
on the console.