Expand default Java .gitignore

The default Java `.gitignore` should include the files generated by each of the popular IDEs, as it's very possible for different developers for an open-source project to prefer different tools.

The additions were mostly taken from the CraftBukkit project: [<code>.gitignore @ ff7c371a81</code>](https://github.com/Bukkit/CraftBukkit/blob/ff7c371a81437ec9f88b1134bcc322308b796a4e/.gitignore). I just reordered them and took out the entries specific to that project.

 - IDE files for Eclipse, Netbeans, and IntelliJ
 - .DS_Store files
 - Maven /target folder
 - Other general build folders: /build, /bin, /dist