Fix warnings with gcc 4.4 in C++11 mode

GCC 4.4 has support for an early draft of rvalue references. When compiling the conforming code it produces warnings such as '../boost/optional/optional.hpp:152: warning: returning reference to temporary'. In order to fix this regression use a special implementation of move(), similar to std::move() on this compiler.
Member fun value() that throws on uninitialized