Bug 834877 part 1.  Add a DOMString struct to use for binding return values. r=peterv
Bug 834877 part 2.  Start using DOMString as the return value for strings.  r=peterv
Bug 834877 part 3.  Add faster DOMString-to-JS conversion code.  r=peterv
Bug 834877 part 4.  Add overloads of GetAttr() and GetId() that take a DOMString.  r=peterv
Bug 834877 part 5.  Add an overload of GetAttribute that takes a DOMString.  r=peterv