Support ndjson -- newline delimited json -- for streaming data.
Hey all,

I'm a developer on [dat project](https://github.com/maxogden/dat) (git for data) and we are building a python library to interact with the data store. 

Everything in dat is streaming, and we use newline delimited json as the official transfer format between processes. 

[Take a look at the specification for newline delimited json here](https://github.com/ndjson/ndjson-spec/)

Does pandas support this yet, and if not, would you consider adding a `to_ndjson` function to the existing output formats?

For example, the following table:

```
> df
    a        b          key                     version
0  True   False  ci4diwru70000x6xmmn19nba1        1
1  False  True   ci4diww6j0001x6xmbyp5o2f0        1
```

Would be converted to 

``` python
> df.to_ndjson()
'{ "a": True, "b": False, "key": ci4diwru70000x6xmmn19nba1, "version": 1}\n{ "a": False, "b": True, "key": ci4diww6j0001x6xmbyp5o2f0, "version": 1}'
```

For general streaming use cases, it might be nice to also consider other ways of supporting this format, like a generator function that outputs ndjson-able objects
