Zolertia Z1 port for RIOT OS

First version of RIOT OS for Zolertia Z1 (platform described in RIOT's Wiki on GitHub).

What should work:
* All functions related to the MSP430F2617 MCU; this includes: timers, UARTs (can communicate on the micro-USB port, and thus have a shell/console), GPIOs (can control LEDs, receive signal from user button...), etc.
* 802.15.4 networking via the CC2420 transceiver.

Todo (in a next version):
* support the real-world sensors (TMP102 thermometer and ADXL345 accelerometer) by creating the appropriate drivers
* support the data/log flash memory (M25P16 -- 16 Mbit Flash chip)