update gradle plugin to 0.7 and use wrapper

Using the wrapper ensures you're building with the right version of
Gradle.

http://www.gradle.org/docs/current/userguide/gradle_wrapper.html
Move to a multi-project gradle build
Set group and version in top-level build file
Merge pull request #91 from loganj/gradle_wrapper

update gradle plugin to 0.7 and use wrapper