Move the default agent behavior to HTTPClient

[Julian]

If you have an object which wishes to make HTTP requests but allow for injecting an alternate object (for testing or customization purposes, say injecting a StubTreq), HTTPClient is seemingly the right place to do so.

It would be nice if the treq behavior to automatically use the global agent was moved to this layer rather than up in treq.api, so that one could default to just HTTPClient() and not need to poke at additional treq internals.

Specifically, it would be nice I think if https://github.com/twisted/treq/blob/master/treq/api.py#L114-L122 was part of HTTPClient's defaults.

(And for anyone finding this, in the meantime, what I think is the thing to do is to instead have objects get treq.request injected into them rather than HTTPClient).