Allow to define accessors on objects.

Allow to define accessors on objects.

Currently one can only define accessors on object templates.  This patch
allows to create accessors on the fly.

These accessors could control access to elements as well.  This element
support is somewhat rudimentary and may require future work (for example,
we probably don't want to convert index into a string.)

Review URL: http://codereview.chromium.org/2123012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4714 ce2b1a6d-e550-0410-aec6-3dcde31c8c00