Replace Maven with Gradle

[nerro]

Advantage is the Gradle Wrapper (https://docs.gradle.org/current/userguide/gradle_wrapper.html), so CI or developer don't need to install build tool and we can control the build tool version inside of repository.

Following tasks should be done:

• add Gradle wrapper binaries
• remove pom.xml and configure build.gradle
• automate release process to Sonatype Nexus with Gradle task