refactoring of TableRowParser::TableCellNode#parse_cellspan()
fixed a bug in OptionManager#set_encoding() in pseudohiki2html.rb
pseudohiki2html.rb: enclosed the last part of code in "if $0 == __FILE__; ...; end"
added test/test_pseudohiki2html.rb
a bug fix: the option --force of pseudohiki2html.rb didn't work properly
added a test in test_pseudohiki2html.rb
removed an unneccesary coditional clause in TableNodeFormatter#visit()
removed a parameter from TableNodeFormatter#each_cell_with_index()
renamed TableNodeFormatter#each_cell_with_index() #each_cell_index()
removed TableNodeFormatter#deep_copy_tree() in markdownformatter.rb
moved TC_OptionManager#set_argv() to the top level
added a test for PageComposer
Merge branch 'converter' into develop-merge-converter

Added tests for pseudohiki2html.rb
Merge branch 'master-refactoring-plainformat' into develop-merge-converter

To reflect refactorings of PlainTextFormat