Possible spurious SettingWithCopyWarning
Found some `SettingWithCopyWarning` warnings in older code with newer pandas. But it's quite possible that these are just side-effects of the warning ('False positives') and it cannot easily be detected, but I am no expert in that field.

Simplified dummy example:

```
In [1]: df = pd.DataFrame(np.arange(20).reshape(5, 4), columns=list('ABCD'))
In [2]: df
Out[2]:
    A   B   C   D
0   0   1   2   3
1   4   5   6   7
2   8   9  10  11
3  12  13  14  15
4  16  17  18  19

# reassign a selection to df
In [3]: df = df[(df['A']%5)!=0]
```

Setting a value with `.loc[]` (this output is from master, in 0.13.1 I got even the advice to `Try using .loc[row_index,col_indexer] = value instead` while I was using it ... in master just the warning):

```
In [4]: df.loc[df['B']==17, 'C'] = 1000
C:\Anaconda\envs\devel\Scripts\ipython-script.py:1: SettingWithCopyWarning: A va
lue is trying to be set on a copy of a slice from a DataFrame
  #!C:\Anaconda\envs\devel\python.exe
```

And then replacing a value with `replace` (also here the advice to use `.loc` is a little bit strange I think):

```
In [5]: df['D'] = df['D'].replace({7:2000})
C:\Anaconda\envs\devel\Scripts\ipython-script.py:1: SettingWithCopyWarning: A va
lue is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  #!C:\Anaconda\envs\devel\python.exe

In [6]: df
Out[6]:
    A   B     C     D
1   4   5     6  2000
2   8   9    10    11
3  12  13    14    15
4  16  17  1000    19
```
