twi: Working code for being able to switch between USCI_B0 and USCI_B1

By just uncommenting the "//#define USE_USCI_B1 1" line, you are able
to use the USCI_B1 module, instead of USCI_B0.

There are also some general I2C improvements, like following the order
suggested in MSP430 Family Guide on how to init I2C. And avoiding
setting stuff in "two_readFrom" and "twi_writeTo" that has already
been set in the "init" method.

There is still need for improvements before the code can be merged.
Ideally, I want to be able to use both modules B0 and B1 from my
sketch, so the Wire/twi needs to support having both modules in
use at the same time.
Fix pinMode_int for UCB1 to contain correct port mapping input
Move twi.c/h into Wire.cpp/h
Add support for two Wire instances, where Wire1 is for using USCI_B1