Make DOMString an alias for Option<~str>

Fixes #898.
Replace our rust_box with std::unstable::raw::Box

This will keep us in sync with compiler changes.  In fact we had the 'prev' and
'next' fields in the wrong order (but we aren't using them).
bindings: Return errors in Result rather than setting an out parameter

Fixes #909.
auto merge of #955 : kmcallister/servo/bindings, r=metajack