Improve test tools and documentation

[vicentealencar]

Currently, the documentation does not specify how to setup the environment and run pyfunct's tests. I believe we must take the following actions:

• Pick a default test runner and add it to setup.py
• Explain how to run the tests in the documentation
• Use travis CI in order to make sure pull requests will not break the build

[gabrielpjordao]

:+1: