Isolated classloading for kotlin gradle plugin

All non-bundled Gradle plugins share common classpath. This may cause some troubles (e.g., see [this thread](http://devnet.jetbrains.com/thread/444525)). Only available solution (for Gradle 1.5+) would be manually resolving dependency conflicts (see [this thread](http://forums.gradle.org/gradle/topics/what_is_the_best_way_to_organize_classloading_of_a_plugin_developed_as_a_standalone_package))

To aviod conflicts, kotlin plugin will try to resolve own core on-the-fly, and load it in separate, parent-last URL class loader (used [this one](http://stackoverflow.com/questions/5445511/how-do-i-create-a-parent-last-child-first-classloader-in-java-or-how-to-overr)).