Fixes issue #472 - memory leaks

*Summary:*  Plugs remaining memory leak, completing fix for issue #472.  

*Intended Effect:* Fix leak in `log_softmax` by using correct operator new in `agrad::vari::operator new`.

I did not need to fix the other possible leaks called out in issue #472 because they were either (a) explicitly qualified to use `agrad::vari::operator new`, or (b) were using an unqualified version within a class extending `agrad::vari`

*How to Verify:*  Run the tests.  To really convince yourself, unplug the leak and see the tests fail with not enough memory allocated on our auto-diff stack because it's allocated on heap.

*Side Effects:* None.

*Doc:*  Not user facing.  I documented the tests themselves a bit.

*Reviewer Suggestions:*  Daniel, because he's going to want to look at the tests.  

And perhaps back out the changes to stack_alloc for efficiency reasons --- I really don't think we need them for this test because we're going to change all of our auto-diff soon anyway.