ipfsApi.Client should use defaults when given None as a value

[larsks]

When host, port, or base is None, ipfsAPi.client.Client should use the default value rather than trying to work with None. This makes the module easier to use; consider, for example, if I have a Python client that includes:

import argparse

def parse_args():
    p = argparse.ArgumentParser()
    p.add_argument('--host')
    p.add_argument('--port')
    p.add_argument('--base')
    return p.parse_args()

I would like to be able to do this...

api = ipfsApi.Client(host=args.host, port=args.port, base=args.base)

...rather than requiring a complex set of conditionals to deal with various combinations of user-specified vs. user-wants-the-default values.