doc: add Sphinx framework for building docs

doc: add Sphinx framework for building docs

I can't get numpydoc working.  Enabling the module raises lots of
warnings:

  $ PYTHONPATH=.. make SPHINXBUILD=sphinx-build-2.7 dirhtml
  ...
  Traceback (most recent call last):tools
    File "/usr/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 321, in import_object
      __import__(self.modname)
  ImportError: No module named TestRFFT
  Traceback (most recent call last):
    File "/usr/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 321, in import_object
      __import__(self.modname)
  ImportError: No module named TestUnitaryRFFT

  /.../FFT-tools/doc/source/mod/FFT_tools.rst:64: WARNING: autodoc can't import/find exception 'FFT_tools.TestRFFT.failureException', it reported error: "No module named TestRFFT", please check your spelling and sys.path
  /.../FFT-tools/doc/source/mod/FFT_tools.rst:64: WARNING: failed to import object FFT_tools.TestRFFT.failureException
  /.../FFT-tools/doc/source/mod/FFT_tools.rst:64: WARNING: toctree references unknown document u'mod/FFT_tools.TestRFFT.addCleanup'
  ...
  None:None: WARNING: toctree contains reference to nonexisting document 'mod/FFT_tools.TestRFFT.addCleanup'
  ...
  <autosummary>:None: WARNING: toctree contains reference to nonexisting document 'mod/FFT_tools.TestRFFT.addCleanup'
  ...