Frame: document "fin" field in the constructor and fin() accessor. to_bytes() now encodes "fin" bit according to the fin field of the Frame object.
Frame: POD update: ATTRIBUTES - type section is moved under new() method. This is way more straightforward.
Frame: add "continuation" frame type. opcode() method is revised so that it can correctly return 0 if $frame->{opcode} == 0.
Frame: now "type" field for new() is immediately resolved to opcode, so that new(type => $type) and is_{type} methods are now consistent. This fixes github issue vti/protocol-websocket#19 ($frame->{type} field is preserved for backward-compatibility, but direct assignment $frame->{type} = "close" followed by $frame->to_bytes() will not work anymore. However, direct assignment is considered rude anyway...)