PION-1325: VS workaround in hash_map.hpp doesn't work for VS 2013
Now uses std::unordered_multimap for _MSC_VER >= 1600 (i.e. VS >= 2010).
Merge pull request #15 in RND/pion from PION-1325 to develop

# By Mignon Belongie
# Via Mignon Belongie
* commit 'dcde86da44edd2a5369a2d0d7e9d28feefea1f96':
  PION-1325: VS workaround in hash_map.hpp doesn't work for VS 2013 Now uses std::unordered_multimap for _MSC_VER >= 1600 (i.e. VS >= 2010).