add decorator for use cassette

    Usage:

       from vcr.decorator import use_cassette

       @use_cassette()  # the default path will be 'fixtures/vcr_cassettes/foo.yaml'
       def foo(self):
           ...

       @use_cassette(path='fixtures/vcr_cassettes/synopsis.yaml', record_mode='one')
       def foo(self):
           ...