Make Maven repository configurable in download_jars

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