bug fix plus conversion to maven plugin

A few changes I needed to make for my project here. I should have committed them separately but am a Git novice. 

The bug fix is in JavaPropertyDeclarationCache, and is fairly self explanatory - the issue was that the implementation code was incorrectly generated in ontologies where there is multiple inheritance, because you were tracking all visited classes rather than classes visited within one branch of hierarchy. so properties were not inherited from multiple parents (only the first discovered). No test cases. Sorry.

The conversion to a maven plugin affects pom.xml, and ProtegeCodegenMojo.java and README.md. In reality these changes could easily be kept as a separate project if it is not desirable to include them in this project

The other changes are fairly cosmetic, and specific to my purposes, but might be worth considering.