Adding support for web client (HTTP::Client).

1. New class HTTP::Client for handling of web client requests and response.
2. Ripple effect on CosaWebClient example sketch.
3. Improved handling of client socket disconnect.
4. Improved handling of program memory strings in Socket/W5100 interface.
5. Fixed problem with isconnected().
6. Renamed W5100::begin with hostname to begin_P.