start encouraging people to use HTTPClient and not module-global stuff

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Treq has a global HTTP client, and maybe that's a good thing for quick experiments, but it makes testing difficult and creates type confusion.

Instead:

1. treq.client.HTTPClient should be promoted somewhere more top-level and central, like perhaps treq.session() or somesuch.
2. the documentation should universally prefer this.
3. the global instance of this thing should be an explicit global variable (treq.default ?) and all the aliases (treq.get, treq.post et. al.) should be deprecated.

Previously: #242 #243