allow customization of "retry" policy.

[adimascio]

When the harvested OAI server returns an HTTP 503, the default policy is to retry 5 times and wait 120 seconds between each try. Due to the variety of OAI server implementations, one might want to configure those parameters. This policy can be customized through the BaseClient.custom_retry_policy's parameter. For instance::

>>> client = Client('http://the-oai-base-url.org', custom_retry_policy={
        # retry on both 500 and 503 HTTP return codes
        'expected-errcodes': {500, 503},
        # wait for 30 seconds before retrying
        'wait-default': 30,
        # retry 10 times
        'retry': 10,
    })

)

[adimascio]

An alternative would be to use https://github.com/jd/tenacity but that would require and external dependency.

[adimascio]

Any chance this PR be reviewed any time soon ?

[jascoul]

Thanks, this looks really great, I've just merged and updated the HISTORY.txt