Better handling of JavaScript exceptions.

Their wrapper has saner message and it actually appears in stacktrace.
Merge pull request #342 from herby/js-exceptions

Better handling of JavaScript exceptions.
- Fixed JavaScriptException to make the tests greem
- Use the native exception message in as the messageText
Use proper CPS in interpreter
Class comments
Renamed ASTDebugger >> stepOver to #step
Merge branch 'master' into DomBinding