WPRichTextView: Fixes "Empty Post" glitch

#### Problem: 
Even if we initialize *WPRichTextView* with a minimum height of 1px, down the road, when autolayout executes, that view gets shrinked to 0px, due to the empty contents of *DTAttributedLabel*.

#### Fix:
Now we're enforcing a minimum height (1px) by constraints. This, in turn, makes CATiledLayer to always render its content, and thus, fixing the glitch we were observing, in which the contents of the post wouldn't render until the user scrolls the view.

Closes #2347 

/cc @aerych