Source tree restructuring

Introduced a whole bunch of changes to the source tree structure:
- There's no separate backend/.  It became the top level mvn project.
- editor/ is now under gui/ (or src/main/resources/org/apache/giraph/debugger/gui/)
- plugin/ is now under eclipse-plugin/

Also did organize imports and source reformatting to some files.

It's now in a pretty good shape to be added as a Maven module to the Giraph source tree.