added interface and documentation to Frame class

[debug-ito]

I added some interface to Frame class mostly because I want to craft WebSocket frames to send in a more fine-grained way.

Some of the added interfaces were already implemented, but they are now clearly documented and tested.

Overview of the changes:

• Document "fin" field for new(). to_bytes() now encodes "fin" bit according to it.
• Add "continuation" frame type (opcode 0). Together with "fin" field, users can now send fragmented frames.
• Document and test "opcode" field for new(). If "type" and "opcode" conflicts, "type" will win, which is documented and tested.
• Document and test opcode() accessor method.
• Now "type" field for new() is immediately translated to opcode. This fixes vti/protocol-websocket#19
• Document "masked" field for new().
• Document "version" field for new().
• Document and test that append($chunk) will modify $chunk, which surprised me.

[vti]

Wow. Nice work!

Don't remember why append is destructive. Probably for ease of work with AnyEvent, so the input buffer is automatically cleared. Maybe this is not a very good idea after all :)