Moving forward we're going to assume `mbstring.func_overload` is off.
For that reason we're not going to call `mb_` functions when checking byte level strings.
Separated handshake negotiation into its own class
`HandshakeNegotiator` deals with Request/Response classes
These changes are geared towards separate responsibility
Refs #29
Updated RFC6455 handshaker to check values case insensitively
Made sure RFC6455 handshaker matches encoding properly
Added mbstring as a requirement for Ratchet
Refs #28, #30