Issue #323 dynamic property assignment

[dsmithhayes]

This Pull Request will address the deprecation of dynamically adding properties to objects, as outlined in this issue. From the Author's suggestion, I have chosen to break the API and add the setTemplate() method into the Interface.

This change will pass all tests and has updated documentation. This will break older versions of the library that rely on the $template property to be public (or non-existent). I am open to looking to a better way to compose the necessity of having the Template class injected into the Func class that further injects into the ExtensionInterface concrete classes.