Issues/46 comments refresh

This branch does a few things:
- Refactors to have only a Comment model. AbstractComment and ReaderComment are removed from CoreData.
- Adds MigrationMapping to wrangle the changes to core data.
- Moves the comment syncing logic into the existing Comment service.
- Adds infinite scroll support to the reader comments. 
- Creates a sync helper class (in swift!) to wrangle syncing state.
- Adds support for estimated row heights and cached row heights to WPTableViewHandler.

Closes #46 
I think it's OK as it is. I doubt there's any performance hit from all the padding, and it's not too confusing, specially after adding some comments.

However, a simpler solution might be to ditch the padding, and use `NSNumericSearch` on `-[ReaderPostDetailViewController fetchRequest]`:

```objc
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"hierarchy" ascending:YES comparator:^NSComparisonResult(NSString *s1, NSString *s2) {
    return [s1 compare:s2 options:NSNumericSearch];
}];
```

A quick proof-of-concept:

```objc
NSArray *array = @[ @"1", @"100", @"1.1", @"1.100", @"1.2", @"2.2", @"1.2.1", @"1.2.100", @"1.2.3"];
NSArray *sorted = [array sortedArrayUsingDescriptors:@[[NSSortDescriptor sortDescriptorWithKey:@"" ascending:YES]]];
NSLog(@"sorted normally: %@", sorted);

NSSortDescriptor *numericalSort = [NSSortDescriptor sortDescriptorWithKey:@"" ascending:YES comparator:^NSComparisonResult(NSString *s1, NSString *s2) {
    return [s1 compare:s2 options:NSNumericSearch];
}];
NSArray *sortedNumerical = [array sortedArrayUsingDescriptors:@[numericalSort]];
NSLog(@"sorted numerical: %@", sortedNumerical);
```

That yields:

```
2014-08-29 19:27:54.962 comparison[71773:303] sorted normally: (
    1,
    "1.1",
    "1.100",
    "1.2",
    "1.2.1",
    "1.2.100",
    "1.2.3",
    100,
    "2.2"
)
2014-08-29 19:27:54.963 comparison[71773:303] sorted numerical: (
    1,
    "1.1",
    "1.2",
    "1.2.1",
    "1.2.3",
    "1.2.100",
    "1.100",
    "2.2",
    100
)
```