pretty printer: avoid to print the ListTerminator at the end of a syntatic list
replaced *.out files with generated outputs
modified the pretty printer to sort the cells of a bag according to the their order in user-declared configuration
updated *.out files in 4_imp++/lesson_6
updated *.out files in imp++/lesson_7
sort multiplicity cells according to alphabetic order; refactor the code to use comparator in sorting
used natural order in sorting map elements; fixed bugs; updated *.out files
excluded Alphanum from checkstyle copyright check; updated LICENSE with the new source code
excluded java backend from ktest config files of imp/imp++
included imp++ back in the regression test
merged with the upstream unified-builtins2
fixed the pretty printer because I forgot to use natural sorting on cells
Merge pull request #540 from yilongli/unified-builtins2

fixing all lessons (except lesson_3) in tutorial/4_imp++/