Kotlin plugin for Google's new android build system

Kotlin plugin for Android build system. Though the system is gradle-based, usual kotlin-gradle plugin won't work and needs to be re-implemented.

Under the hood, new plugin waits for full project evaluation and collects all build variants (produced by google's plugin). Then, Kotlin compilation task is properly configured (including android sdk libraries, generated java sources and other classpath elements) per each variant and injected into tasks graph.

User still needs to include compile dependency on "kotlin-runtime" (or better, on "kotlin-stdlib") - see sample project here: https://github.com/nskvortsov/kotlin-examples/tree/master/gradle/android-mixed-java-kotlin-project