improve logging and add method entry/exit instrumentation

Here's a first take at some improvements to logging, per discussions with @marco-c in IRC.

This changeset adds the logConsole and logLevel URL params, where logConsole can be any comma-separated combination of *web* (the Web Console), *page* (the in-page console), and *native* (the native console, i.e. *dump*); and logLevel can be any of the values *trace*, *log*, *info*, *warn*, *error* (in that order of severity). Messages at or above the specified log level will be logged to the specified consoles.

I also went through the various `console.*` calls and changed their severities where appropriate. And I added a primitive mechanism to instrument method entry/exit, so we can log within non-native methods without having to modify their Java source. I'd like to add some tests, although I'm not yet sure how to implement them.

Yes, I completely agree, let's do that as a followup!
I would really love a filtering mechanism, that's what I really miss.