Support .trust_env attribute on the client instance. #185

We should add a trust_env attribute and __init__ keyword argument to BaseClient.
This should be used as the default if trust_env is not explicitly specified on the request.

The default value for trust_env on the per-method calls should then switch to None, so that we can distinguish it from "explicitly True" and "explicitly False".

This'd be a nice enhancement on top of #177.