nsqd: /mput forces line oriented data formats

**NSQ** is supposed to be data format agnostic.

We initially overlooked this shortcoming of `/mput` to make it backward compatible with `simplequeue`.

I propose we add a request parameter (`?binary=true`) that *enables* binary mode for `/mput` and requires a request body of the same format as the TCP `MPUB` command:

```
<# messages><msg size><msg bytes><msg size><msg bytes>...
```

This would allow the next release to be backwards compatible but support the ability for users to begin upgrading publishers in anticipation of us dropping support for `\n` delimited bodies in `1.0` (ping #126).

cc @jehiah