Made Provider API sample more idiomatic for what we can do with Kotlin now
Made Provider API sample more idiomatic for what we can do with Kotlin now

This takes the approach to make Kotlin look more like the explicitness
of Java until we can do something fancier (like setter overloads).

- We want to expose a single getter that returns the Provider/Property
- The GreetingExtension and Greeting task had unnecessarily mutable variables
- Handwritten separate setters/getters for a Property isn't what we want people
  to do.  And I think we don't want authors to expose special methods for
  setting Provider or raw values.

Fixes #597

blindpirate/gradle,gradle/gradle,robinverduijn/gradle,gradle/gradle-script-kotlin,gradle/gradle,gradle/gradle,gradle/gradle,robinverduijn/gradle,robinverduijn/gradle,blindpirate/gradle,gradle/gradle,robinverduijn/gradle,robinverduijn/gradle,blindpirate/gradle,robinverduijn/gradle,robinverduijn/gradle,blindpirate/gradle,blindpirate/gradle,robinverduijn/gradle,blindpirate/gradle,gradle/gradle,blindpirate/gradle,blindpirate/gradle,gradle/gradle,gradle/gradle,gradle/gradle,robinverduijn/gradle,blindpirate/gradle,gradle/gradle,gradle/gradle-script-kotlin,blindpirate/gradle,robinverduijn/gradle,robinverduijn/gradle