with extensive assistance from @cdent

this should make debugging a little less painful

oddly, this only seems to work for STDERR - @cdent?

because why not

before:

    gabbi-run -r my_response_handler:MyResponseHandler

after:

    gabbi-run -r my_response_handler

the latter is *much* more convenient

not sure where to document the contract though: the given module is
expected to contain a `gabbi_response_handlers` object, which is either
a sequence of response handler classes or a function returning such a
sequence

/cc @cdent

By default testr will swallow output to STDOUT so if we want to make
output of gabbi-run visible when there are unexpected errors from
runner.run() we need to send it to STDERR.

(For those inclined this change can be tested by changing a test in
test_runner to fail and then `tox -epy34 -- test_runner`.)

not exactly sure what this means, but @cdent suggests these should be
linked now