Support count queries

[mikemintz]

Will ParseReact support Parse count queries in the observe function?

See https://parse.com/docs/js/guide#queries-counting-objects

[agnosticdev]

Instead of using the count query I have been getting the length on the array of objects returned from the observe function.

observe: function() {
   return {
      comments: (new Parse.Query('Comment')).ascending('createdAt')
   }
}

render: function() {
   //get count of your return data
   console.log(this.data.comments.length);
  ...
}

[andrewimm]

@mikemintz We've been trying to work out a decent way to expose this expanded functionality on ParseQuery. Most of the trouble is a matter of working out an ideal API. Ultimately, we need to generate a count Observable from a query, and also decide whether counts will be automatically updated by mutations. Once we have a decent proposal for the API, I think we should move forward with implementing just that bit, and we can treat local updates as improvements to be dealt with down the road.

[TylerBrock]

It would be great to support count so that higher level components don't have to query for entire objects.

Can we merge #88?

[andrewimm]

Re: #88, @philikon and I were hoping to have a conversation around maintaining updated stores, but I think the consensus is that we will not dynamically update count queries for now. Hopefully we can land it in the next few days.

[philikon]

I finally found the time to update #88 with the changes that @andrewimm and I talked about offline.