IO Converters
discussed in #4698

`convert_data` would return an appropriate Converter object to do various types of conversions. Mainly useful when you have to chunk both sides of these.

```
csv -> hdf
sql -> hdf
hdf -> csv
```

```
c = pd.convert_data('csv','hdf', chunksize=....)
c.read_csv(input_path. ......)
c.to_hdf(output_path, key.....)
c.execute()
```

pretty simple under the hood...
biggest issue is MAY need to read the input file twice (for read_csv) (if ints are detected in the first input chunk), because the dtype MAY change (e.g. you get a NaN later in later chunks)

useful?
