Allow use of default socketio Server

[TheJedinator]

It may be desired (as in my case) to not use the AsyncServer, could allow an argument in the constructor to choose which server type to use.

[adamlwgriffiths]

If you look at the amount of code in this library, it really amounts to about 4 lines of code.
It would be more pragmatic to just modify the file to what you want.