Tests: add ability to check which Mandrill API endpoint was used.

Add DjrillBackendMockAPITestCase.assert_mandrill_called; use it in representative backend test cases.

(Also make get_api_call_data work with various ways of calling requests.post.)
Add send-template tests (and fixes).

Add test cases for send-template.
Expand template_content dict into Mandrill's name/value array.
Don't send template_content as "None" if missing.

Clean up some variable names in the backend.
Readme: docs for send-template

(and break up the lengthy 'usage' section)