Add timeout to all Requests calls

Use a default timeout of 30 seconds for all requests, and add a
REQUESTS_TIMEOUT Anymail setting to override.

(I'm making a judgement call that this is not a breaking change in the
real world, and not bumping the major version. Theoretically, it could
affect you if your network somehow takes >30s to connect to your ESP,
but eventually succeeds. If so, set REQUESTS_TIMEOUT to None to restore
the earlier behavior.)

Fixes #80.

anymail/backends/mailjet.py

@@ -115,7 +115,7 @@ class MailjetPayload(RequestsPayload):
         if template_id and not self.data.get("FromEmail"):
             response = self.backend.session.get(
                 "%sREST/template/%s/detailcontent" % (self.backend.api_url, template_id),
-                auth=self.auth
+                auth=self.auth, timeout=self.backend.timeout
             )
             self.backend.raise_for_status(response, None, self.message)
             json_response = self.backend.deserialize_json_response(response, None, self.message)