Add default filename for download

[dopplershift]

Right now, Dataset.download() requires a filename to be passed in. We should make that optional, defaulting to the value of Dataset.name. So download should look roughly like

    def download(self, filename=None):
        if filename is None:
            filename = self.name
        with self.remote_open() as infile:
            with open(filename, 'wb') as outfile:
                outfile.write(infile.read())

with of course an updated docstring.

[dopplershift]

Closed by #232.