Gradle plugin

This patch contains initial revision of kotlin-gradle-plugin.

The plugin allow Gradle to build mixed kotlin-java projects without additional configuration.
Kotlin compiler confiuration is missing at this stage.

gradle_api_jar - gradle subproject that produces maven artifact with Gradle API. This artifact must be installed to accessible repository before building kotlin-gradle-plugin. To install it to local repository, one can run `gradlew install` in this directory