Fix grouped df in dynamic view

In light of #71, we should push grouped data spec in initial spec so updates on source data could flow in consequent updates. This would also address #154 and #150.

The changes are that we always attempt to push an initial version of data and defs during spec creation. In consequent updates with `ggvis_data` message, we only updates the "source" data from `message.value[0]`. Note for grouped data, `message.name!=message.value[0].name`.

I think the following example verifies that this fix works as data can be updated dynamically:
https://gist.github.com/saurfang/11d2cc1c6c041bcfa721
```
runGist("11d2cc1c6c041bcfa721")
```