added PlainTextFormat.format()
added HtmlFormat.create() for consistent access to PseudoHiki::*Format classes
added PseudoHiki::Format
added sinatra_helpers.rb
Merge branch 'sinatra' into develop

defined a helper method for Sinatra.
an ugly bug fix of htmlplugin.rb
Merge branch 'htmlplugin' into develop
added sinatra_helpers.rb to pseudohikiparser.gemspec
refactoring of TableRowParser::TableCellNode#parse_cellspan()
fixed a bug in OptionManager#set_encoding() in pseudohiki2html.rb
Merge branch 'develop' into ME-task-specific

reflecting updates and bug fixes from development branch