[V8] Add hashing and comparison methods to v8::String

[V8] Add hashing and comparison methods to v8::String

This allows us to more rapidly search for a v8::String inside a hash
of QStrings.

Change-Id: Ied9625aeefba730816d9ed7aa609a36592e5fc33
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>