Added Symbol primative PropType.

With the edition of ECMA-262, we now have a new primative type called
Symbol. This primative type should be added to the PropTypes, as users
will eventually be using Symbols in their props for describing unique
and immutable data, such as identifiers.

src/isomorphic/classic/types/ReactPropTypes.js

@@ -73,6 +73,7 @@ var ReactPropTypes = {
   number: createPrimitiveTypeChecker('number'),
   object: createPrimitiveTypeChecker('object'),
   string: createPrimitiveTypeChecker('string'),
+  symbol: createPrimitiveTypeChecker('symbol'),
 
   any: createAnyTypeChecker(),
   arrayOf: createArrayOfTypeChecker,