SettingWithCopyWarning for string manipulation in dataframes with a non-monotonic index
Hi,

I get a SettingWithCopyWarning for string manipulation in dataframes with a non-monotonic index. 

```
SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
```

This only happens if the integer index skips a value. The code to reproduce:

```
from pandas import DataFrame
# create example dataframe:
df = DataFrame ({'column1':['a', 'a', 'a'], 'column2': [4,8,9] })
df
# assign string to 'column1':
df['column1'] = df['column1'] + 'b'
df
# it works just fine - no warnings
#now remove one line from dataframe df:
df = df [df['column2']!=8]
df
# adding string to 'column1' gives warning:
df['column1'] = df['column1'] + 'c'
df
```

NB: this only happens after a fresh login into python; if you run it again, it will not appear any more (!)

System:
- Pandas 0.13
- Python 2.7.2
- Ubuntu 12.04LTS 64bit

Is this is an actual issue or just a nonsensical warning?

Kind regards, 
Egle
