prettify_file.rb: added support for multiple files, improved help, applied minor refactorings, and renamed to prettify_xml_file.rb.
Simplified pretty_print_rows and fixed one bug in it.
Added Gemfile.
Added 'byebug' gem to Gemfile (development group).
Added Cell concept, which is now used to store the values. Styles are currently not supported.
Added styles support to Document (no Table/Row/Column level).