Make Maven repository configurable in download_jars

[msinto93]

Currently python setup.py download_jars is hardcoded to download the KCL jars from the public Maven repository https://search.maven.org

def package_url(self, group_id, artifact_id, version):
        #
        # Sample url:
        # https://search.maven.org/remotecontent?filepath=org/apache/httpcomponents/httpclient/4.2/httpclient-4.2.jar
        #
        prefix = 'https://search.maven.org/remotecontent?filepath='
        return '{prefix}{path}/{artifact_id}/{version}/{dest}'.format(
                                        prefix=prefix,
                                        path='/'.join(group_id.split('.')),
                                        artifact_id=artifact_id,
                                        version=version,
                                        dest=self.package_destination(artifact_id, version))

https://github.com/awslabs/amazon-kinesis-client-python/blob/master/setup.py#L118

Can we make this configurable, say via an environment variable (where we default to the current behaviour if not set)? This would allow this package to be installed directly via a pip install for anyone using their own Maven repository. I'm happy to propose a PR for this, just looking to gauge some thoughts first.

[msinto93]

Alternatively (or as well as), if you could release Python wheels for the package alongside the source distributions that would also resolve this.

[nlm4145]

This is something that should still be considered in the implementation.