API: add astype coerce argument / remove raise_on_error
http://stackoverflow.com/questions/25952790/convert-pandas-series-from-dtype-object-to-float-and-errors-to-nans/25952844#25952844

maybe remove`raise_on_error` and rename to `coerce` (more inline with the rest of pandas API)

```
In [30]: pd.Series([1,2,3,4,'.']).convert_objects(convert_numeric=True)
Out[30]: 
0     1
1     2
2     3
3     4
4   NaN
dtype: float64
```

I suspect this might make a nice API as well

```
s = pd.Series([1,2,3,4,'.'])
s.astype('float64', coerce=True)
```

maybe `coerce=True|None|'raise'` make sense
