Ellipsize long titles in the posts list on Reader

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. 

UPDATE: Updates made to the pull request. The ellipsizing is now done based on height of the label, not character length. The ellipsize method has been updated to use CFStringTokenizer to account for languages without spaces between words.