Fixtures, reusable state for writing clean tests and more.

Fixtures defines a Python contract for reusable state / support
logic, primarily for unit testing. Helper and adaption logic is
included to make it easy to write your own fixtures using the fixtures
contract. Glue code is provided that makes using fixtures that meet
the Fixtures contract in unittest compatible test cases easy and
straight forward.

Note: There is a cyclic dependency between the testtools package and
the fixtures package.  Since these are python libraries, one has to
just install both of them.