Split protobuf.home property into include and bin

This allows mvn to reference the homebrew installation, rather
than a relative one.
merge ScottDugas: Split mvn property protobuf.home property into protobuf.include and protobuf.bin

Merge pull request #1314 from ScottDugas/sql-layer/homebrew-compatibility
https://github.com/FoundationDB/sql-layer/pull/1314