Converted SimpleITK into a package from a simple module

Due to the way pip installs packages flat, and wheels' different
handling of data_files, the setup.py now installs SimpleITK as a
package which import all for the SimpleITK.SimpleITK sub-package. The
non-normal handling of installation of setup.py is preserved, along
with the namespace usage of SimpleITK.

Change-Id: Id9588061bac5b7d702a62f569886e3b3cc1f2e84
add creation of wheels to the dist target

Also download and install the python wheel package into the virtual
environment.

Change-Id: Ia240fb34a35b2de97b147da27097fd7f3e8de683
Add CMake Option to build Python wheels for dist target

Change-Id: Ibbcd6624a2c1f1f4e5b5ffc336a4bf8d2888deee
The absolute paths of the doc files are not working with setuptools

They are getting munged and being modified causing errors during
packaging.

Change-Id: I0f87fbf094fedea0666801d6eb3b4d38ad622981
Merge topic 'SIMPLEITK-506_Refactorsetup.pyForWheels' into next

4830ed88 The absolute paths of the doc files are not working with setuptools
518b755c Add CMake Option to build Python wheels for dist target
c5dbc0ec add creation of wheels to the dist target
d5b6f6d6 Converted SimpleITK into a package from a simple module