Implement String.prototype.codePointAt and String.fromCodePoint.

Contributed by Mathias Bynens <mathiasb@opera.com>.

TBR=mathiasb@opera.com, rossberg@chromium.org
BUG=v8:2840
LOG=Y

Review URL: https://codereview.chromium.org/406863003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22493 ce2b1a6d-e550-0410-aec6-3dcde31c8c00