Prepare kotlin-gradle plugin for Gradle 2.0

Prepare kotlin-gradle plugin for upcoming Gradle 2.0 release.

As recent Gradle 1.12 is announced to be the last release of 1.x branch, it is time to clean things up. There were several deprecation warnings produced by the plugin, that will turn to errors in 2.0

Unfortunately, fixing these warning also requires upgrading Gradle API , making plugin incompatible with versions below 1.6

**NOTE**
I do not have permissions to deploy artifacts to `repository.jetbrains.com/util` and I can not deploy new gradle api artifact myself. So after merging the pullrequest, someone must navigate to 
`libraries/tools/kotlin-gradle-plugin-core/gradle_api_jar`
and execute `gradlew build && mvn deploy` command