Switched to rust libcore

The code still requires cleanup, for now main.rs re-exports all
required std symbols.
Fixed travis build.
Fixed demo code using libstd.
Added missing lang items and cleaned up the reset.
Changed all 'use std::' to 'use core::'.
Added todo note for std::cmp.
Merge pull request #35 from farcaller/back-to-core

Switch to rust libcore.
Detached GPIO from GPIOConf (as it should have been in the first place).
Added basic gpio interrupt handling.
Bad ugly code for IRQ handling.
Added basic mrf driver and test application. Compiles but no tx/rx code yet.
Added sending code, demo now can talk to other devices.
Added basic irq handler.
A bit cleaner code, still not really the best option. Also, huge.
Updated for libcore.