Added pin names for mbed board

That code doesn't actually work, but I'll let it be in there for some
time, so that I can stop looking into mbed schematics.
UART's "no parity" is now Disabled instead of None.
Added more useful APIs to GPIO

 * level() returns input gpio level;
 * set_direction() changes gpio direction.

This code makes use of #[deriving], binding zinc to libstd.
Added DHT22 driver and demo app.