fix #13: incompatibility with Gradle 1.8

* Creating a separate branch 'gradle-1.8' for gradle 1.8 compatibility.
* Upgraded the gradle wrapper and related files to gradle 1.8.
* Changed the 'groovy' configuration reference 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
Merge pull request #14 from mbjarland/gradle-1.8

fix #13: incompatibility with Gradle 1.8