#692 Add the possibility to specify "Sec-WebSocket-Protocol"

[KaSSaaaa]

I took the liberty to add some documentation as well. Feel free if you have any feedback :)

[MichaelSB]

I could use your split function in my websocket compression PR, if yours is merged first :D

[KaSSaaaa]

I rewrote find_first_of algorithm and put it in utility.h :)

[MichaelSB]

If I don't miss it, I think a getter is needed for the selected subprotocol in the connection. Otherwise looks good to me.

[KaSSaaaa]

Currently, the "subprotocol_" is only a private variable and is only used in the Connection::start but I can add a getter in the interface and implementation if it's needed.

[MichaelSB]

I think that's it :) I asked for the getter, because users must be able to know which subprotocol was selected and probably switch code paths accordingly.

[gittiver]

build fails on ubuntu at least, other os will run later.

[KaSSaaaa]

Apparently missed the include of for utility::find_first_of ...

[mrozigor]

May I ask you also to add unit tests?