General beast improvements and fixes:
* New maybe_const_t alias for maybe_const
* New safe_bool using C++11 features
* Fix is_call_possible metafunction:
  - Works on empty argument lists
  - Works with reference types
beast::asio improvements
* New enable_wait_for_async for safe cleanup
* New memory_buffer, a managed asio compatible buffer
* shared_handler improvements:
  - Can be 'empty' (no stored handler).
  - Default constructible as 'empty'.
  - Safe evaluation in bool contexts, false==empty