Add requests style `history` attribute to response.

See: http://docs.python-requests.org/en/latest/user/quickstart/#redirection-and-history
Fix weirdness in other test.
Only want to build the history when we have actually got a response.
Fix test broken by change to addCallback. Order results properly.
PEP8 kgo.
Merge branch 'master' into add-response-history

Conflicts:
	treq/client.py
	treq/test/test_client.py
Move history() to the Treq Response API and don't calculate it until it's needed.
Fix pep8
history() raises NotImplementedError on older twisted versions.
comma comma comma comma comma chameleon.
Document redirects some.
Merge pull request #53 from dreid/add-response-history

Add response history