Add an option for standalone Asio

[wakaztahir]

Standalone Asio is being used in a lot of web sockets projects along with Boost.Asio with conditional compilation, We should also provide an option for using it.

In my pull request, I have replaced Boost.Asio with Standalone Asio, I will make commits to use Boost.Asio so users who use boost don't need to do anything and users who want to use Standalone Asio can do so by defining an option.