Miscellaneuos changes thanks to Dan Preston

Miscellaneuos changes thanks to Dan Preston
* Improved Makefile (libDAI now also builds out-of-the-box on MacOSX,
  thanks to Dan Preston; merged Makefile.win and Makefile.shared into Makefile)
* Fixed bug in calcMarginal, calcPairBeliefs, calcPairBeliefsNew where
  states with zero probability mass were clamped, leading to NaNs or assertion
  errors (thanks to Dan Preston for reporting this)
* toc() now returns POSIX system time with maximum accuracy of microseconds
* Exception objects now remember their error code