TelosB (MSP430x1) I2C improvements ported from Steve Ayer's version for shimmer platform

This is a small modification on Steve Ayer's I2C driver for the shimmer platform to meet the requirements by the telosb platform implementation.
I think this fits now for all MSP430x1 but I can only check on telosb and in in the Thread

http://mail.millennium.berkeley.edu/pipermail/tinyos-devel/2012-October/005191.html

is an open discussion if Ayer's version should populated in tos/chips/msp430.

I think that this version fits not for all msp430 chips. Because of that i have added it in the telosb platform which uses an MSP430x1.
Tested with LM75 temperature sensor. Working fine.