sliceJoin replaced with flatSelect
A new expression dsl system is introduced to allow easy 2-stage selection: page -> element -> text/attribute
join has one more parameter section to allow insertion of data after flatten, this fully enabled LINQ-style query lambda that has a left-right-select format. Thanks a lot Liu Chen!
aggregate implicit view bound into 2 packages: dsl for those used in dsl, view for those aren't.
minor refactoring: e.g. PageBuilder -> Session, PageSchemaRDD -> PageRowRDD
rename test module to integration
moved several unit test
examples are no longer abused as integration test.
example has more parameters