Enable Python threading by unlocking the GIL

Simply by passing the "-threads" flag to SWIG, the wrapping code will
now unlock the GIL during each C call. Currently, SimpleITK does not
have call back or event monitoring. Threading will make these features
more complicated.

Change-Id: Ia97913c1a9577a24e8d5cb481c4d9ec66ca32cc6
Merge branch 'SIMPLEITK-349_RemoveGitHashFromNighlties' into SIMPLEITK-348_PythonThreading

Change-Id: Ib91127c89fa90e288421928371cdf68bfb71c990
Removing dead code from numpy test

Change-Id: Ia86aa2f3e49cccbc3c019d7a7e9fc8e72df748da
Merge topic 'SIMPLEITK-348_PythonThreading' into next

63dc1d3 Removing dead code from numpy test
932491e Merge branch 'SIMPLEITK-349_RemoveGitHashFromNighlties' into SIMPLEITK-348_PythonThreading
e9ce728 Enable Python threading by unlocking the GIL