NSFormatter and NSDecimalNumber support

* Added support for NSDecimalNumber to XLFormViewController and XLFormTextFieldCell
* Added formatter related properties and methods to XLFormRowDescriptor
* Updated XLFormTextFieldCell to use formatters when specified.
* Created example page for formatters, FormattersViewController, and linked from OthersFormViewController
* Added SHSPhoneComponent to Podfile to demonstrate a phone formatter on FormattersViewController.