Implements ES6 String.prototype.normalize method.

BUG=v8:2943
TEST=Unit tests for "real life" use cases, edge cases, various types of normalization.
TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/60013010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17532 ce2b1a6d-e550-0410-aec6-3dcde31c8c00