use treq for sandbox http resource
get the content in `_make_success_reply`
make DummyResponse behave like treq expects
better tests, lower level HTTP client stubbing
check StringIO() values
unused import
setting content-length header on DummyResponse
better files param format
do not default to empty dict for files
data limit tests (thanks @justinvdm)
include received bytes in error reason (thanks @justinvdm)
remove auth parameter (thanks @justinvdm)
dont assume headers is a dict
fix tests for header param change
treq 0.2.1 is what we need, thanks @dreid!
Fix docstring typos.
Merge branch 'develop' into feature/issue-735-http-resource-multipart-posting

Conflicts:
	setup.py
	vumi/application/tests/test_sandbox.py
Add inlineCallbacks lost in merge.
Merge pull request #735 from praekelt/feature/issue-735-http-resource-multipart-posting

sandbox http resource needs to support multipart post