ci-phpunit-test for CodeIgniter 3.0

I'm planning to merge the PR: https://github.com/kenjis/ci-phpunit-test/pull/74

It changes $this->getDouble() method.

If you have any trouble with your existing test cases when you apply the patch,
please let me know.

The PR initially only contains the change of $this->getDouble().
But after all, I got to the conclusion that the reset_instance() implementation was wrong.
So I added the fix.

I've just merged the PR, so now 1.0.x@dev (dev-master) includes BC break changes.

See https://github.com/kenjis/ci-phpunit-tes...md#changed
for details.

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