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.
Updated readme
Fixed misnamed getter in gpio code.
Can't copy Cell of std, so using init() instead.
Fixed typo.
A quick hack to source globals from env. Fixes #16.
Remove default_handlers.S
Merge pull request #13 from bharrisau/simple_isr

Remove default_handlers.S.
Remove extra line from lpc/iomem
Merge pull request #18 from bharrisau/lpc_bug

Remove extra line from lpc/iomem
Initial support for travis-ci
Merge pull request #19 from bharrisau/rust-ci

Initial support for travis-ci
Added travis-ci badge
Allow full memory access in gdb by default.
Remove deprecated transmute_mut call. Fixes #20.
Cleaning up the build toolchain, round 1

 * better dependency tracking;
 * smaller Rakefile, configurable things moved to external yml files
 * split most of rake.rb mess into dedicated files.
Fixed yaml dependency name.