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.

djrill/tests/__init__.py

@@ -1,3 +1,4 @@
 from test_admin import *
 from test_legacy import *
 from test_mandrill_send import *
+from test_mandrill_send_template import *