Cleanup build instructions and scripts

[aaltat]

The Python build and release has changed since the original Selenium2Library build instructions and scripts has been written. Cleanup the scripts to support modern ways.

Included tasks:

• Better build docs
• Clean up setup.py
• Remove ez_setup (done as separate issue Remove ez_setup #646)
• Clean up MANIFEST.in
• Clean up Remove setup.cfg
• Better build scripts (probably based on Invoke like with RF itself)
• Remove references to already deleted doc directory
• Remove references to already deleted demo directory

[pekkaklarck]

Edited by @zephraph to move checklist to the ticket description for more visibility

[zephraph]

Hey @pekkaklarck, I moved your checklist up to the description that way it can be seen from the issues list.

[emanlove]

I would recommend we also upgrade the test runner (which could be another ticket).

[pekkaklarck]

Agreed. I would assume Python 3 adds some new requirements for test runners too. This is also something where we can look for inspiration (or steal code) from RF. If there are lot of very similar needs, we may want to extract some build and test scripts to a separate shared project. Anyway, enhancing test runners definitely needs a separate task.

[pekkaklarck]

All checkboxes in the description checked.