Updates needed in "Pushing to pip" section of "packages" tutorial

[paxtonfitzpatrick]

in addition to changes re: TestPyPI from #68:

• command to build package should be changed to to python setup.py sdist bdist_wheel so wheel binary is also created (see PEP 427)
• encourage running twine check dist/* before uploading
• upload command should be changed to twine upload dist/* so .whl file is available on PyPI
• replace link to article at bottom with link to this tutorial instead