Build and publish wheels in CI

[cirla]

It would be nice if travis built and published manylinux wheels to cut down on issues related to building from source distribution.

See https://github.com/pypa/python-manylinux-demo for example.

[cirla]

I have a work in progress to do the following:

1. Switch from Cython to cffi so we can build the library with a C99 compiler but can still build the extension for Python 2.7 with VC9. This would allow tulipy support Python 2.7 on Windows.
2. On tags, have Travis/Appveyor build and publish wheels to PyPI for all combinations of:
   • Arch: i686 and x86_64
   • OS: Windows, OS X, Linux (manylinux1 and manylinux2010)
   • Python: cpy27, cpy37, pypy

[cirla]

Going to drop all support for Python 2 as it reaches end-of-life in a month. We can stay with Cython.

Also not going to pre-build x86 wheels, just x86_64.

On tags, have Travis/Appveyor build and publish wheels to PyPI for all combinations of:

• OS: Windows, OS X, Linux (manylinux1 and manylinux2010)
• Python: cpy37, cpy38, pypy