BUG: Fix brittle pivot margins
closes #3334  Brittle pivot margins

Issue was that pivot tables that use all columns of the original DataFrame in rows and cols failed on the margin computation. This is a special case for margins: one should use the index itself as the value.

```
>> df = DataFrame({'Response' : ['Y', 'N' ,'N', 'Y', 'Y', 'N'],
            'Type' : ['A', 'A', 'B', 'B', 'B', 'C']})

>> pivot_table(df, rows='Response',cols='Type',aggfunc=len,margins=True)
>> Type      A  B   C  All
>> Response               
>> N         1  1   1    3
>> Y         1  2 NaN    3
>> All       2  3   1    6
```

This is my first PR to Pandas, so apologies for anything out of the ordinary.

pls move the release notes to 0.13.thanks! (you prob need to rebase too)
 I moved release note to v0.13.0.txt and rebased the branch. 
 can you squash into 1-2 commits?

`git rebase -i <commit 1 prior to yours>`...then s/f
 sorry..should have mentioned that before
 Fixed
 also...need to rebase on master (what happens is there are conflict on release notes....no biggie)....just rebase and i'll merge you right away
 woops! Turns out my origin master was out of date. Should have used upstream. Hang on, will fix.
 @jreback I made a new branch off master and cherry-picked my commits into it. I picked up a change in a test file
https://github.com/pydata/pandas/blob/master/pandas/tools/tests/test_pivot.py#L11
that makes my tests fail. Any idea what could have happened there?
 you need to rebase this

something like

updates master to current

```
git checkout master
git pull
```

```
git branch my-branch --set-upstream master
git checkout my-branch
git pull --rebase
```

will bring you up to current
 Fixed - sorry.
 @jreback for some reason I made the decision to never use pull (I like to use fetch/merge fast-forward/rebase so I don't get in sticky situations) but now I look again it looks like you can do that with pull too, cooool

```
git pull --ff-only
```

_not sure I'll change._
 @jreback Travis is green. Any idea when this can get merged in?
 @cpcloud can u take a look?
 @guyrt thanks! merged in
 via 69f559447bc2ac8dc216aad3569dbaf6f46280eb
