better error message for to_datetime
cc @danbirken

```
In [2]: to_datetime([1,'1'])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-1d4cd9e078aa> in <module>()
----> 1 to_datetime([1,'1'])

/home/phillip/Documents/code/py/pandas/pandas/tseries/tools.pyc in to_datetime(arg, errors, dayfirst, utc, box, format, coerce, unit)
    137         return Series(values, index=arg.index, name=arg.name)
    138     elif com.is_list_like(arg):
--> 139         return _convert_listlike(arg, box=box)
    140
    141     return _convert_listlike(np.array([ arg ]), box=box)[0]

/home/phillip/Documents/code/py/pandas/pandas/tseries/tools.pyc in _convert_listlike(arg, box)
    117                 result = tslib.array_to_datetime(arg, raise_=errors == 'raise',
    118                                                  utc=utc, dayfirst=dayfirst,
--> 119                                                  coerce=coerce, unit=unit)
    120             if com.is_datetime64_dtype(result) and box:
    121                 result = DatetimeIndex(result, tz='utc' if utc else None)

/home/phillip/Documents/code/py/pandas/pandas/tslib.so in pandas.tslib.array_to_datetime (pandas/tslib.c:16487)()

TypeError: object of type 'int' has no len()
```
