Fix setting _p_changed when removing from small Python BTrees/TreeSets.

[jamadden]

Sadly, the fix for #11 was incomplete (sorry) and failed to handle removal, which bit us testing our application under PyPy. This PR handles removal and adds test cases for it.

Example test failures before the fix:

======================================================================
FAIL: testRemoveInSmallMapSetsChanged (BTrees.tests.test_OOBTree.OOBTreePyTest)
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Traceback (most recent call last):
  File "//BTrees/BTrees/tests/common.py", line 1179, in testRemoveInSmallMapSetsChanged
    self.assertTrue(t._p_changed)
AssertionError: False is not true

======================================================================
FAIL: testRemoveInSmallSetSetsChanged (BTrees.tests.test_OOBTree.OOTreeSetPyTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "//BTrees/BTrees/tests/common.py", line 1421, in testRemoveInSmallSetSetsChanged
    self.assertTrue(t._p_changed)
AssertionError: False is not true

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Twice, out of many test runs, I saw the following test failure for Python2.7. I'm pretty sure it's unrelated:

======================================================================
ERROR: test_LP294788 (BTrees.tests.testBTrees.BugFixes)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "//BTrees/BTrees/tests/testBTrees.py", line 355, in test_LP294788
    id = trandom.choice(list(ids.keys()))
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/random.py", line 275, in choice
    return seq[int(self.random() * len(seq))]  # raises IndexError if seq is empty
IndexError: list index out of range

----------------------------------------------------------------------

[jamadden]

It turns out that plain xSet objects have the same issue as the Tree versions did in #11. I added a fix and tests to this PR.

[tseaver]

Don't we also re-fix #11? Should be mentioned here (so that folks who experienced the bug in 4.1.2 know to expect that it is fixed).

[jamadden]

Issue 11 was about adding to the Tree versions; that didn't change here. This issue handles adding to the non-Tree Sets, as well as removing from Trees. I'm not sure how to reword the change entry to make it more clear, but I'm open to any suggestions.

[tseaver]

OK, I do see that it mentions the sets.

[tseaver]

I'm good to merge this once the changelog is updated.

[tseaver]

Released via: https://pypi.python.org/pypi/BTrees/4.1.3