Added implementation in PSTCollectionViewFlowLayout for:
- (void)encodeWithCoder:(NSCoder *)coder
Writes out the same keys that are used in initFromCoder.  This fixes an issue where if the flow layout was serialized and then deserialized it wouldn't
contain the original values.
Merge pull request #277 from jeff96/master

Fix for Issue: #276 - encodeWithCoder:(NSCoder *)coder in PSTCollectionViewFlowLayout is not implemented