Packaging update

[crowellel]

Hi all. These changes are in response to issue #518 and bring the package to minimal compliance with packaging standards so we don't get deprecation warnings from pip due to outdated packaging format. Highlights:

• Changed packaging and uploading commands in REAME to allow for building and uploading a source distribution as well as a wheel.
• Added language to setup.py to create universal wheel (necessary for continuing py2.7 compatibility - even if py2.7 is EOL).
• Add minimal pyproject.toml file to meet standards.

This is my first PR, so if I missed something please let me know!

[wookayin]

This seems to be only for a particular PEP517 choice python-build. Shouldn't we have pip install build somewhere?

[crowellel]

I hadn't thought of adding that, but will if you believe it's necessary. I don't think it's really necessary for two reasons:

1. This is only for building the package for distribution - not for general end-user installation or re-installation. The build package is not actually required for installing our package.
2. We're using the twine library a few lines below and don't mention pip install twine for that either.

I chose this method because it brings us into compliance with pep 517 with minimal effort and almost no change to the package as a whole.

At some point, someone will probably want to do some more work to move a lot of the code from the setup.py file to the pyproject.toml file, but this at least makes us compliant and will stop pip from complaining every time someone installs the package (as it is doing now).

[justinmk]

Thank you for the help here! We definitely need more owners for pynvim, it's suffering from a lack of attention.