Remember ArticleView text size between app launches

Solves issue #230
Clean UnifiedDisplayView from irrelevant things
Fix UnifiedDisplayView text sizing

ArticleCellView must have the makeTextLarger, makeTextSmaller, canMakeTextLarger and canMakeTextSmaller methods.
We send these methods calls to one of the visible ArticleCellView.
Fix canGoForward (irrelevant)
Merge pull request #291 from barijaona/bugfixes

Improve "Bigger Text/Smaller Text" command