De/serialize streams

Provides streams to serialize and deserialize Tile and Info objects. In practice it looks something like:

```javascript
// Serialize from a tilelive source
var src = createTileliveSource();
tilelive.createReadStream(src, {type: 'scanline'})
    .pipe(tilelive.serialize())
    .pipe(fs.createWriteStream('/some/file'));

// Deserialize to a tilelive sink
var dst = createTileliveSink();
fs.createReadStream('/serialized/file')
    .pipe(tilelive.deserialize())
    .pipe(tilelive.createWriteStream(dst));
```

Uses line-delimited JSON as a serialization format. This may or may not prove to be a significant bottleneck but seems okay in some simple benchmarks.