1) Add new type JSProxy for representing proxy objects.
   Currently devoid of functionality, i.e., all properties are undefined.

2) Some rudimentary global $Proxy functions to create proxies.

Next step: Hook up getProperty and getOwnProperty handlers. Will probably
require introducing a new LookupResult type, which is a mixture of
INTERCEPTOR (handles any property) and CALLBACK (calls back to JS).
Can we unify this somehow?

TODO: Should probably rename existing Proxy type to something like
"Foreign", to avoid confusion.

Review URL: http://codereview.chromium.org/6932068

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7887 ce2b1a6d-e550-0410-aec6-3dcde31c8c00