Allow modules to be disabled from local.xml.phpunit

Uses the following XML structure:

    <phpunit>
        <disable_modules>
            <Mynamespace_Mymodule/>
        </disable_modules>
    </phpunit>
Documentation fixups.  Primarily around _isAllowedModule method.
Merge pull request #126 from aligent/feature/disable_modules

Allow modules to be disabled from local.xml.phpunit