Add data persistence for notes.

[Hussain Rizvi]

Todo's:

• Currently the notes are being stored in browser's local storage.
• What we can do is add github authentication using oAuth.
• This will allow us to call github apis for the authenticated user.
• Using that we can create or fetch any repo for a authorized user.
• Each user who authorize through github will have a repo created for notes storage on his account.
• This repo will store json file containing data about notes and categories of notes.