Store refresh token in Folder Picker Sample App

The refresh token is stored in the keychain to enable
the folder picker to remain authorized between app
restarts.

To initiate an automatic refresh, the app includes a
heartbeat API call to fetch the root folder's
information.

This commit also adds a logout button to the main view.

Closes GH-3

Signed-off-by: Ryan Lopopolo <rjl@hyperbo.la>
Fix problem with Archiving

The solution that worked for me was to add
a header search path that includes the SDK

Closes GH-4

Signed-off-by: Ryan Lopopolo <rjl@hyperbo.la>