Improving IOStream interface; adding write_P() and using write() as base.
Improving Socket interface; using IOStream::Device as base class.

1. Allow Socket to be used as a IOStream::Device and bind to IOStream.
2. Default implementation of write/read as send/recv.
3. Removed available/room as these are included in base class.
Major update of W5100 device driver; allow binding to IOStream.

1. Use internal TX buffer as write buffer when binding to IOStream.
Efficient message construction without processor buffers.
2. Add new members for Socket and IOStream::Device interfaces.
3. Updated CosaWebServer to illustrate the improved stream to socket.