Refactor number and cvc directives

* BREAKING: change $type to $ccType for card types
* BREAKING: separate expected and actual card types
* eliminate use of isolate scopes
* $watch attribute value (expression)
* $parse attribute value in $validators
* Add better documentation for type discovery/expectation
Remove comment about optional type="number" with expiration directives
Split expiration docs into clearer sections
Revert broken link that pointed to creditcards

38ea82208fd8abfda3f88533afe6e0016a9efd0b
Release v2.0.0