Generate c++ code with std::shared_ptr instead of boost::shared_ptr.

Most modern compilers now have full support for std::shared_ptr when enable with c++11 flags. It would be nice to have the option to generate code that uses this instead of boost::shared_ptr. This would enable us to remove another boost dependency, on the road to a dependency-free thrift library

Solution summary (so you don't need to wade through the comments):

A new header, <thrift/stdcxx.h>, creates the namespace apache::thrift::stdcxx and aliases the proper std:: or boost:: constructs depending on the capabilities of the environment and whether any build flags are used to force an override of this behavior. All code throughout the project now uses stdcxx:: for all smart_ptr and functional (bind, function, placeholders, _1, _2, ...) constructs.