fix #13: incompatibility with Gradle 1.8

* Creating a separate branch 'gradle-1.8' for gradle 1.8 compatibility. Note that a version compiled with 1.8 will be incompatible with earlier versions. 
* Upgraded the gradle wrapper and related files to gradle 1.8.
* Changed the 'groovy' configuration reference in build.gradle to 'compile' as per deprecation instructions from gradle (the groovy configuration will be removed in gradle 2.0 and is deprecated now)
* Upgraded the plugin version to 1.2.2 as we are introducing an incompatibility