Maintainance release

Multiple bugfixes.

Some new features:

• Boost.Log is made optional.
• Logging to std::clog is supported.
• Logging can be disabled, and the log-level set at compile time. Use RESTC_CPP_LOG_LEVEL_STR cmake option.
• The asio context can be multi-threaded (excellent for use in servers, and to stress-test servers). Enable RESTC_CPP_THREADED_CTX cmake option.

Updated to work with the latest Debian, MacOS and Windows 10 OS/compilers.