Graceful timestamp fallback

Let me cite the commit message of the main commit to describe this change

    if a timestamp for a give file is not found, add the current time as
    timestamp to make the browser always reload the file from the server and
    not serve a cached version.
    
    In case a file is requested, that is not in the files array, it is
    impossible to tell the browser not to use a cached version. If the file
    has been changed meanwhile, the tests will still run with an old
    version, which might be unwanted. The user is informed about this by a
    warning in the development console if the file is not in the base path.
    A timestamp is added in this case. If the file is located within the
    base path, the user is informed by an error message and no timestamp is
    appended.
    
    It is a valid use-case to not have files in the files array, that should
    be served. The files might be served by a proxy server (karma itself
    provides the possibility to specify a proxy server for a specific
    directory). It might not possible for karma to detect any changes in the
    files, served by the proxy. Therefor no caching should be done for these
    files and the timestamp should always be attached.