Make the task 'build' (and 'assemble') produce a fat JAR

Previously, the 'gradle build' command did not produce a fat JAR,
which is not usable in the arena (missing dependent libs).
This commit modifies build.gradle so that the 'assemble' task depends on
the 'fatJar' task, making sure that a fat JAR is produced.
Add build instructions to README.md

The correct way to build the plugin is `gradle build`.
Merge pull request #74 from wookayin/gradle-fatjar

Modify Gradle build script to produce a fat JAR.