Properly remove the TSOfferViewController and TSShareViewControllers

### Bug

Current TSWordOfMouthController does not remove the TSOfferViewControllers and TSShareViewControllers when the Word of Mouth controller is dismissed. This results in both a memory leak as well as potential instability issues in ios 8.

### Fix
1. Implement best practice methodology for adding and removing childViewControllers [Apple documentation](https://developer.apple.com/library/ios/DOCUMENTATION/UIKit/Reference/UIViewController_Class/index.html#//apple_ref/occ/instp/UIViewController/childViewControllers)
2. Remove unnecessary passing of parentViewControllers into childViewControllers