Added MarkIndependent to the persistent handle API.  Independent handles are independent of all other persistent handles and can be garbage collected more frequently.

Implemented the get trap for Harmony proxies.  Proxies are enabled with the --harmony-proxies flag.

Performance improvements and bug fixes on all platforms.

R=ager@chromium.org

Review URL: http://codereview.chromium.org/7043003

git-svn-id: http://v8.googlecode.com/svn/trunk@7923 ce2b1a6d-e550-0410-aec6-3dcde31c8c00