Using a connection buffer to send RspBuilder.

This implements #206.

The type of `connBuffer` is now `Buffer.Buffer` (not blaze-builder's `Buffer`). I made this change because `RspBuilder` needs `Buffer.Buffer`.

Blaze-Builder's `Buffer` is created in `RspSource` on demand. It is possible to contain blaze-builder's `Buffer` to avoid the overhead of creation. But I think it is wise to make `Connection` independent from blaze-builder.

@snoyberg please review this.