Enable caching in UnifiedDisplayView
Changes in UnifiedDisplayView

- simplified start of Webview handling
- add article's row management in ArticleCellView
- change the way we detect a non relevant cell (check our article's folder Id and row)
- fix Webview error handling
- fix an exception
Small changes to try to limit flickering in UnifiedDisplayView

- remove all setNeedsDisplay tricks : MacOS X probably knows better than me
- standardize initial size of cells
"Clean" and standardize cells before their reuse by UnifiedDisplayView
Optimized UnifiedDisplayView

- adjust management of ArticleCellView's inProgress property
- store and use as soon as possible the pieces of information used to detect non relevant cells
- remove strange tricks in -listView:cellForRow
- no wait time when dimension problem is detected
Precaution against UnifiedDisplayView flickering

Finer management of rowHeightArray, when a bottom cell get finished before upper cells.

Conflicts:
	src/UnifiedDisplayView.m
Changed UnifiedDisplayView error handling

webView:didFailLoadWithError:forFrame: and non relevant cell
Implement PXListViewCell's layoutSubviews method

This seems to solve efficiently the "shifted cell" problem.
So we remove many unneeded tricks.
Merge pull request #261 from barijaona/UnifiedDisplayViewFix

UnifiedDisplayView improvement : smoother scrolling, less mispositioned cells.