Update pom.xml

- Switched from spout-repo to oss-sonatype
- Added bukkit-repo
- Updated to maven compiler 3.1
- Updated to Bukkit/SpoutcraftPlugin 1.6.4-R2.1
- Remove redundant version

Signed-off-by: Steven Downer <grinch@outlook.com>
Update commands

- Added /backpack clear <player> command
- Made /backpack reload and /backpack clear <player> use the backpack.admin permission node

Signed-off-by: Steven Downer <grinch@outlook.com>