add maven central sync requirements on pom.xml

To put this plugin to maven central repository, pom.xml must satisfy some requirements: [Central Sync Requirements](https://docs.sonatype.org/display/Repository/Central+Sync+Requirements)

I like this plugin and I want this plugin to upload on Maven Central Repository!

After this pull request merged...
You can upload this plugin on central repository yourself: [Uploading Your Jar to Maven Central](http://kirang89.github.io/blog/2013/01/20/uploading-your-jar-to-maven-central/)
...or I can done it myself: [Uploading 3rd-party Artifacts to The Central Repository](https://docs.sonatype.org/display/Repository/Uploading+3rd-party+Artifacts+to+The+Central+Repository)