Add maven build (fixes #83)

[ben-manes]

The maven build can be run using: mvn clean install and outputs to target. This includes building the source jar, so that IDEs can resolve that during debugging.

This change makes the dependency available on Jitpack. It will build the commit on-demand (fixed, branch, or snapshot) and cache the results. This way the dependency can be sourced without you needing to push a new build.

For example, to fetch the build (log) from my fork would require users adding:

<repository>
  <id>jitpack.io</id>
  <url>https://jitpack.io</url>
</repository>

<dependency>
  <groupId>com.github.ben-manes</groupId>
  <artifactId>icafe</artifactId>
  <version>-SNAPSHOT</version>
</dependency>