Mode added to BusIn + allow creation of NC pins

Now BusIn can also use PullUp, etc, instead of only BusInOut.

If the pin is NC, it does get to the init, but all write/mode functions
are disabled. This is how it used to be in the old gpio version. Quite
some libraries allow users to make pins NC, and they are all locking up
with the current mbed version. This is far from a perfect solution, but
more a temporary fix.
Merge pull request #247 from Sissors/master

Mode added to BusIn + allow creation of NC pins