API/BUG: inconsistent return in Timestamp/to_datetime for current year
after #3890, still unresolved / inconsistent
non-trivial to fix, so pushing to 0.12

```
In [14]: Timestamp('2012')
Out[14]: Timestamp('2012-06-18 00:00:00', tz=None)

In [15]: to_datetime('2012')
Out[15]: Timestamp('2012-01-01 00:00:00', tz=None)
```

```
In [17]: pd.date_range('2014', '2015', freq='M')
Out[17]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-04-30, ..., 2015-03-31]
Length: 12, Freq: M, Timezone: None
```
