Fixes for detection of <unordered_set> vs. <tr1/unordered_set>, 64-bit Darwin.
  Hello! This is so cool looking, I can't wait to try it out. I ran into an
  immediate little problem though, the defines you were testing to determine
  whether to use <tr1/unordered_set> vs. <unordered_set> were wrong on my
  machine, and when I looked at it I saw there were a few other cases that
  it would be returning the incorrect header, especially on Darwin.
  Let me know if for some reason you think this is wrong, but I'm pretty
  confident that it now covers the various cases correctly.

  Also, I saw in the samples makefile how you were trying to correctly
  detect 32- or 64-bit on Darwin, which can be very tricky, because some
  versions of Mac OS X which are in fact fully cabable of running 64-bit
  binaries run a 32-bit kernel and Unix subsystem. So I went ahead and
  covered up the gaps there, if thats ok!

  :)

~Geoff