prevent CSRF exploits

Getting the necessary `crumb` is easy:

    def get_crumb_data(jenkins_url):
        crumb_issuer_url = jenkins_url + '/crumbIssuer/api/python'
        data = urlopen(crumb_issuer_url).read().decode()
        crumb_issuer_response = literal_eval(data)
        crumb_request_field = crumb_issuer_response['crumbRequestField']
        crumb = crumb_issuer_response['crumb']
        return {crumb_request_field: crumb}

But currently `jenkinsapi` seems to be unable to pass it along with each request.