support for CompositeTemplates

As documented in https://www.docusign.com/p/RESTAPIGuide/RESTAPIGuide.htm#REST%20API%20References/Send%20an%20Envelope%20from%20a%20Template.htm

Needed in order to send a template with pre-populated values.
We are posting a single template here. According to rest guidelines, the path should be singular i.e. 'template'