featu(computed): Add computed property

This PR adds support for computed properties. 

It addresses https://github.com/platanus/angular-restmod/issues/85

Todo: 

- How is masking handled? - In most (all?) cases these properties should not be sent to the API. Does the user need to define the mask, or should we add the mask automatically? 
- The performance of these computed properties is not optimized since there is no watching. The function is run every time the property is accessed. This will only be noticeable in huge quantities.