update README for interface change in WPYPaymentViewController
add travis build status badge in README
remove reference to model in AbstractCardField
pass parameter to setup in WPYAbstractCardField
define and declare setText: WPYAbstractCardField

a setter method that fires textFieldChanged is required
in subclasses
group methods in WPYAbstractCardField
rename cardValue to rawCardValue in WPYAbstractFieldModel
adjust WPYCvcField to change in abstract class

- remove method remove from superclass
- adjust to signature change
- rename method into appropriate name
- delegate logic into model

- define class method as instance method
adjust WPYExpiryField to changes in abstract class

- remove methods removed from superclass
- update signature/rename method
adjust WPYNameField to changes in abstract class

- remove methods removed from super class
- update signature/rename methods
adjust WPYNumberField to changes in abstract class

- remove methods removed from super class
- update signature/rename methods
[bugfix] change setCardValue in WPYExpiryField

It did not parse expiry correctly when value was
incomplete.
[refactor] use existing method to avoid repetition 
in WPYCvcFieldModel

- rawCardValue for self.card.cvc
- isAmex for getting brand and comparing string
[refactor] use rawCardValue instead of card.getter
in WPYNameFieldModel
fix failing test due to interface change of WPYCvcFieldModel
add test for setCardValue: in WPYExpiryFieldModel
define and declare invalidFormWithError: in 
WPYCardFormView
[refactor] declare getter for selected expiry instead
of holding expiry as property.
replace old delegate method in WPYPaymentViewController
edit README.md