AutobahnPython doesn't print critical error messages by default

While debugging several issues with Autobahn (including #445), I realized that critical information that would help track down the problem was being hidden by default. Errors like these should be visible to the user by default:

Failing WAMP-over-WebSocket transport: code = 1011, reason = 'WAMP Internal Error (resolve() takes exactly 2 arguments (1 given))'
WAMP-over-WebSocket transport lost: wasClean = False, code = 1006, reason = 'connection was closed uncleanly (I failed the WebSocket connection by dropping the TCP connection)'

Instead, I didn't see them until I found the debug, debug_wamp, and debug_app parameters in the API Reference. It's also not clear what the distinction is between them, or why they should all exist. With the simple test script I used in #445 there is no output at all from Autobahn without specifying debug_wamp=True and the application simply appears to hang.

Start with the API Reference entries for debug, debug_wamp, and debug_app, then reproduce the simple test script described in #445. Trace why the critical transport errors are hidden by default and clarify the distinction between the parameters. Done means the reported errors are visible by default and the parameter behavior is clear.