patching the setup for Mac OS X

Tested on OS X 10.8.3, Python 2.7.5, hdf5 1.8.11 (homebrew)
Merge pull request #331 from SnippyHolloW/master

Mac OS X setup adaptation
Allow zero-length selections
When slicing yields an empty array, return an emtpy array of the correct shape
Tests for slicing returning empty arrays