Include the library in open source Sonatype/Maven central for ease of distribution

Hi Nayuki, I changed the package hierarchy to correspond to the maven dependency structure. I added maven pom so a jar can be built. Jar can now be fetched at Maven central and Sonatype.

https://oss.sonatype.org/content/groups/public/com/github/nayuki/BMP-IO/

Thanks,

Tommy