Add lag parameter to autocorrelation
Add lag parameter to autocorrelation, default to lag-1 autocorrelation so existing code will work unchanged.

This is not good! Failure unrelated to my commit:

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FAIL: test_tquery (pandas.io.tests.test_sql.TestXMySQL)

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Traceback (most recent call last):
  File "/home/travis/build/pydata/pandas/pandas/io/tests/test_sql.py", line 2238, in test_tquery
    tm.assert_series_equal(result, expected)
  File "/home/travis/build/pydata/pandas/pandas/util/testing.py", line 674, in assert_series_equal
    assert_almost_equal(left.values, right.values, check_less_precise)
  File "testing.pyx", line 58, in pandas._testing.assert_almost_equal (pandas/src/testing.c:2556)
  File "testing.pyx", line 93, in pandas._testing.assert_almost_equal (pandas/src/testing.c:1794)
  File "testing.pyx", line 140, in pandas._testing.assert_almost_equal (pandas/src/testing.c:2390)
AssertionError: expected -1.00016 but got -1.00015, with decimal 5

Have you seen this before?
 @omtinez pls add a release note 0.16.0 enhancements section. and then squash. looks ok otherwise.
 ok this seems ok. pls rebase on master and squash. ping when green.
 Sorry about the mess, new to Git. I squashed the changes and now both autocorrelation-lag and master in my repo should reflect all of my desired changes.
 @omtinez looks like you may have closed the pull request on accident. Could you reopen it?
 For some reason, github closed my pull request and now sees zero commits in my branch. I tried doing a git push -f just to see if github would pick up my commit then but no dice: https://github.com/omtinez/pandas/commit/25b6b3747dd9835ad18350ea4717e91a16619a72

Github won't let me reopen unless it sees the commit, is there a way to fix this or do I need to open a new pull request?
