Fix long titles

This is to fix #1047. 

Summary of the fix: 

1) A new `NSString` category method is created for ellipsizing: `stringByEllipsizingAtLength:preserveWords`
2) Relevant unit tests are created in `NSStringHelpersTest.m`
3) Changes made to `ReaderPostView` class to ellipsize long post titles when `showFullContent` is `NO`. 
4) Changes made to `ReaderPostDetailViewController` and `ReaderPostTableViewCell` to update the method calls for `heightForPost:withWidth` to add `showFullContent` parameter.