Tidy up after reading contributing docs

[nameloCmaS]

This pull request does the below:

1. Tidies the contribute docs generally (updated some out of date links, added some more detail to code examples, updated RST syntax to use code-block in place of highlight descriptor, corrected tox test environments).
2. Changed Firefox example path from canary to firefox_nightly.
3. Removed old rst file referencing install of PyQt4 for now unused driver.
4. Removed link to non-existent rst in old releases.
5. Updated Selenium keys as example code does not work as mentioned in issue #1147.
6. Corrected tutorial to ref second btnK named element as didn't work.
7. Updated is_current function to use property decorator.
8. Added missing lint environment to tox.ini

For item 8 I have rewritten the to.ini file but will raise an issue as a bigger change.