Fixes fetchPosts remote service

- Fetch Post API should send the path via GET instead of POST. This is the reason why the backend call is failing, and we're not getting the associated Post's json.

- ReaderPostService is not saving fetched posts, which causes the reader to render blank posts.

Ref: Issue #1814 

/cc @astralbodies + @aerych 

(Thanks Erick for helping me out!!)

@astralbodies hi there Aaron! Would it be ok with you if we instead replace that call with the following call?:

```
[[ContextManager sharedInstance] saveContext:self.managedObjectContext];
```

Reason is that this method is being used in several places, and it (may not be) obvious that you need to perform the save OP by yourself, after the fetch operation finishes.

(Thanks!!)