Modify .gitignore to not ignore gradle-wrapper.jar

With *.jar also gradle/wrapper/gradle-wrapper.jar is ignored.
build/**/*.jar would be better, but build/ is already excluded.
Add gradle-wrapper.jar to repo
Simplify Gradle wrapper configuration

Requires Gradle 1.7+
Move version related project configuration to gradle.properties
Change version to 0.0.2-SNAPSHOT

It is better to not keep a non snapshot version in a Git repo