Added scalesItemToFill property

It appears that NHBalancedFlowLayout scales item sizes up to fit the row width while preserving their aspect ratios. This produces different row heights, which makes total sense for displaying images.  In my project, however, I am laying out text contents and having a fixed row height would make more sense. Therefore, I added a property `scalesItemToFill` that acts similarly to UIViewContentModeScaleToFill. It changes the aspect ratios of items' preferred size to fill a fixed row height (provided by `preferredRowSize`).

### Before
<img src="https://www.dropbox.com/s/9s1z487p3p3obun/iOS%20Simulator%20Screen%20Shot%20Nov%2016%2C%202014%2C%2010.28.13%20PM.png?dl=1" alt="before" width=350>

### After
<img src="https://www.dropbox.com/s/wih7zsk8ommnu9b/iOS%20Simulator%20Screen%20Shot%20Nov%2016%2C%202014%2C%2010.28.30%20PM.png?dl=1" alt="after" width=350>