implement localstorage

[ghost]

today i was looking through polymer documentation and i came across an interesting element named app-indexeddb-mirror that seemed perfect to work with datatable solutions. but it couldn't be used with paper-datatable because of the way it handles data is using promises and not plain json data. so i thought maybe implementing it inside datatable might be a better choice. saves last list of objects received to localstorage and so when user is offline and has no access to server it would show last values received from server. On promise rejection it would be cool if a bar appears above data table that says "you are currently offline" and shows previous data received. I think it would work great with spirit of progressive web apps that should work in offline mode too.

[dhrytsenko]

@aryaei it's a good point and l think we start to work on it later