Update scientific.rst

Description about Numba has been added.
Update scientific.rst
Merge pull request #281 from ugurthemaster/master

Update scientific.rst
Removes completed TODO re: scientific Python
Adds Sphinx to requirements.txt
Correct spelling of 'aforementioned'.
Nitpicky rewordings and grammar fixes.
Add line breaks for conformance with the Style Guide.
GtK --> GTK
'many' --> 'much'
copy edit structure.rst
Merge pull request #283 from geraldarthur/master

copy edit structure.rst
PEP and Python are capitalized.
'async' --> 'asynchronous'
Code block type is 'console', not 'bash'.
Added note about OSX-GCC-Installer and Xcode conflict.
Fix link to Dive Into Python 3
minor formatting changes.
Rewrite section on choosing a Python version
Merge pull request #287 from untitaker/python2vs3

Rewrite section on choosing a Python version