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
Add ci_char_traits
Add HTTP protocol and client support:
* New http::client_session for making requests
* Add 10,000 popular URL data set for unit tests
* Restructure joyent message parser (from Node.js):
  - New http::raw_parser wrapper
  - Convert parser errors to error_code
  - Enumeration and strings for parsed HTTP method
  - Move parser engine into joyent namespace
  - Rename includes to be distinct