Add a scope object to ensure that leaving a literal scanning prematurely
will clean up after itself.
Also reset the literal buffer if a scanner is reinitialized with a new
source code.

Review URL: http://codereview.chromium.org/3137037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5327 ce2b1a6d-e550-0410-aec6-3dcde31c8c00