introduce POJOs API in core and move proto stuff to dedicated module

This compiles but is incomplete and certainly does not pass any test.
implement serialization of JournalEntry for RaftLog

* use plain old Java serialization
implement properly equals/hashcode/toString
convert from POJOs to Protos and back
added minimal package documentation
remove dependency on protoc and protobuf
Merge pull request #62 from foldlabs/move_protobuf

Introduce POJOs API in core and move proto stuff to dedicated module