Added Evernote login authentication.

Allows users to sign in to, and use an Evernote sandbox account.
Updated Readme with Evernote Authentication details.
Added login button.

This closes issue #10
Added 'log out' button
Changed button links
Passed errors to callbacks.
Merged in change to plain-text
Refactored getNoteStore to be more clear.
Merge pull request #23 from wdoug/evernote_authentication

Allow users to sign in to their own (sandbox) accounts - Closes issue #10