Added a number of handle scopes to the debugger code to keep handles local to the function using them.

Fixed SetDebugEventListener to actually unregister when passed a NULL pointer. Previously this NULL pointer was wrapped in a Proxy.

BUG=1242702
Review URL: http://codereview.chromium.org/21347

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1269 ce2b1a6d-e550-0410-aec6-3dcde31c8c00