Support listing templates and their associated documents.

[mikalstill]

Simple support for listing templates and their associated
documents. An example usage is:

for template in list(client.list_templates()):
    print 'Template %s, %s' %(template['templateId'],
                              template['emailSubject'])
    for document in client.list_template_documents(
          template['templateId']):
        print '    Contains document: %s' % document['name']

[zebuline]

Thx @mikalstill for this PR.
Could you add some tests ? For example, in this file: https://github.com/novafloss/pydocusign/blob/master/demo/templates.py