GenericReader: simplify local stream copy optimization

This pull-request simplifies the changes introduced by #39.  Especially the changes to the algorithms in `GenericReader` are back to plain `s.*` syntax.  This will help me to submit my pending local patches. ;-)

`rapidjson.h`:
 * `StreamLocalCopy`: add default argument to copy optimization selector based on `StreamTraits` of `Stream parameter`
 * drop `operator->`, `operator*`
 * make `Stream` (reference) member public
 * drop empty destructor

`reader.h`:
 * add local references, initialized from "copy" (reverts algorithmic bodies back to plain 's.xx()')