Should fix gcc debug error in tranvis. May need further refactoring.

add `Key()` method to the `Handler` concept

It was a copy-n-paste error for the last argument of `Key()` and `String()`...

For more details see: https://github.com/miloyip/rapidjson/issues/132

This commit tries to minimize the required code changes and forwards the `Handler::Key()` calls to `Handler::String()` wherever possible in order to not break existing code; or at least not code deriving from `BaseReaderHandler` when implementing a custom `Handler`.

Initial C++11 move support

GenericValue: accept values with different allocators for read-only access

This shall generate best possible precision (if strtod() is correctly
implemented). Need more unit tests and performance tests. May add an
option for accepting precision error. Otherwise LUT in Pow10() can be
reduced.