testtools 2.0.0 is incompatible with fixtures 1.2, and due to pip's lack of a
resolver this leads to silently incompatible installs.

Capturing the warnings module output (which is typically used
for deprecating code or functions or modules) is quite useful and
is a frequent operation that can be required to perform. So provide
a fixture that is similar (but not the same) as the warnings
``catch_warnings`` context manager that can be used to gather all
warnings emitted and allows people to later analyze them to ensure
they are as they expect.

Provide tox configuration to make it easy to run tests locally against
different python environments without have to install dependencies
into the system. The default env is left at py27 only for
compatibility, however tox -e py33 and tox -e py34 will work fine with
this in place.

PEP 475 led to ``time.sleep()`` not being interrupted when a received signal
handler eats the signal (rather than raising an exception). (Robert Collins)

When working on OpenStack in tree functional test things like the
context information is extremely useful to have access to. This relies
on using custom log formatter, which currently can't be done in
fixtures.

The creates an additional optional parameter for FakeLogger to specify
this.