Switch Build System to Maven

I've changed the build system from Ant to Maven.  This involves:
* adopting the maven project structure -- lots of files have moved,
* removing many of the dependency jars -- these are now resolved at the run of the "mvn package" build command,
* slight updates to some source -- accounting for the OwlAPI changes and GWT generated code visibility of an enum, and
* updates to the documentation to reflect the new build system.

The maven build supports all the targets of the previous Ant build (albeit renamed) except the build number increment.  There is a version number in the maven build, which I've set to 0.1.  If recreation of the build number is necessary, I'm happy to do it.

This is a significant change to the project structure and build tooling, so I can understand if you want to keep to what currently works.  I simply offer this as an alternative.