Optionally dump API requests and responses to stdout, to simplify
debugging of the raw API communications. Currently implemented only
for Requests-based backends.
This (undocumented) setting can log things like API keys, so is not
appropriate for use in production.
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.
