DatetimeIndex.tz_localize ignores 'ambiguous' keyword
``` python
In [120]: pd.Timestamp('2014-10-26 01:01:20').tz_localize('Asia/Irkutsk')
---------------------------------------------------------------------------
AmbiguousTimeError: Cannot infer dst time from Timestamp('2014-10-26 01:01:20'), try using the 'ambiguous' argument

# ok
In [121]: pd.Timestamp('2014-10-26 01:01:20').tz_localize('Asia/Irkutsk',ambiguous=pd.NaT)
Out[121]: Timestamp('2014-10-26 01:01:20+0900', tz='Asia/Irkutsk')

In [129]: x=pd.DataFrame([1],index=[pd.Timestamp('2014-10-26 01:01:20')])
     ...: x.index
Out[129]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-10-26 01:01:20]
Length: 1, Freq: None, Timezone: None

# so this should work?
In [130]: x.index.tz_localize('Asia/Irkutsk',ambiguous=pd.NaT)

AmbiguousTimeError: Cannot infer dst time from Timestamp('2014-10-26 01:01:20'), try using the 'ambiguous' argument
```
