Updated mailer guide

Added instructions to use the url helper from the mailer. padrino/padrino-framework#1032
Update guides/application-helpers.textile
Improved nested attributes docs

Added more docs that explicitly tell how attributes fields are named and references DataMapper solution (that doesn't come out of the box). Perhaps we should add references to all supported ORMs?
Update guides/controllers.textile

in case "get "/other/:id" the url is generated as "/admin/other/#{params[:id]}", not "/admin/#{params[:id]}"
Merge pull request #18 from DavidToca/patch-1

Update guides/controllers.textile, fixed url generated description in case  'get "/other/:id"'
Added docs on why the server starts on localhost

...and how to easily listen on all interfaces in the system.
Update guides/development-commands.textile