Update plugins generator script and its documentation 

Update the plugin generator script, to generate plugins with  CamelCase or snake_case phrases,and fix the pluralization issue.
The issue was, after creating a plugin with plural term(i.e. 'rails g plugin Thing' as mentioned in USAGE file)
when starting rails server it popup an error saying  
'config/application.rb:81:in `require': cannot load such file -- thing (LoadError)'