 Add tox.ini to start using tox (#1035)
 Add tox.ini to start using tox (#1035)

This commit adds support for using tox by adding a tox.ini. This makes
it very simple for users and contributors to run tests and style checks
locally by just using the `tox` command. This is also a common tool for
python projects, as many (if not most) other python project uses it.

Fixes #1012
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py