[FEATURE: horizontal ListView] ListView now handles `isHorizontal` property

The `Ember.ListView` and `Ember.VirtualListView` handles horizontal list now.

- Added `isHorizontal` property, default to `false`, on which depends the whole feature
- Added `totalSize` and `size` corresponding to `totalWidth` or `totalHeight` and `width` or `height` depending on the new property
- Moved `scrollTop` to `scrollOffset` which is the top or left offset depending on the new property
- Moved `pullToRefreshViewHeight` to `pullToRefreshViewSize` also depending on the `isHorizontal` new property
- Fixed all existing new unit test to follow the refactors
- Removed unused variables and calls to some getter in some methods
- Wrote the same unit tests but for the horizontal version.