Allow passing in multiple tag names to the validation decorator.
    
    We have problems when we try to wrap one decorator around another.
    There are ways to do that with the
    [decorator](https://pypi.python.org/pypi/decorator) module nicely,
    however.. they are ugly.
    
    Take a look at this:
    
    https://github.com/micheles/decorator/blob/master/src/decorator.py#L217-L218
    
    And this:
    
    https://github.com/micheles/decorator/blob/master/src/decorator.py#L185-L188
    
    The approach in this PR is.. simpler.