make the default bidi attribute as true for websocket session type
update doc, add description for websocket server type and bidi attribute for change_type
Merge pull request #46 from onlychoice/update_doc

update doc, add description for websocket server type and bidi attribute...
Merge pull request #47 from onlychoice/ws_default_bidi

make the default bidi attribute as true for websocket session type