to_json() Flat Output
The to_json() I/O command current provides the following output:

```
split   dict like {index -> [index], columns -> [columns], data -> [values]}
records list like [{column -> value}, ... , {column -> value}]
index   dict like {index -> {column -> value}}
columns dict like {column -> {index -> value}}
values  just the values array
```

It would be handy if the output could also be provided in the format:

```
mdarray    [ [index1,value1a,value1b,...] , [index2,value2a,value2b,...], ... ]
```

This format is, for instance, used by the DyGraphs JS library for plotting data.

The following command provides the output I am looking for:

```
np.column_stack((df.index,df.as_matrix())).tolist()
```

(where `df` is a DataFrame)
