Added support for checking coding standards of docs

This is an extension of the work covered in Issue #476, which covered a test of coding standards within Iris. This extension allows Python files within the Iris docs to also be tested.

As docs are not a required part of Iris this extension includes a check to determine whether they are installed or not and sets a flag dependent on the outcome of that check that is referred to later in the test. As many of the docs' Python files are currently not coded to pep8 standards a majority of the changes here adds a second list of files to be skipped by the test, as in the first PR.