Upgrade to the latest server protocol.

Server protocol changes:
* All server requests use Basic access authentication
* There is no more X-Action. Server paths are used instead:
	- /send for sending messages, with the destination address
		in the X-Destination header.
	- /ok for doing nothing. Used for the main activity login status.

Other minor changes:
* Centered the resultTextView in main.xml.
* Utils#parseHttpMessage can parse messages like "(text)".