Implement bluemix client constructor method

[smithsz]

Other than the cloudant_bluemix CM, the only way to construct a bluemix client is as follows:

service = CloudFoundryService(vcap_services, instance_name)
cloudant_session = Cloudant(
    service.username,
    service.password,
    url=service.url,
)

We should abstract this in a class method, something like this:

cloudant_session = Cloudant.bluemix(os.getenv('VCAP_SERVICES'), 'instance-name')