Harvest: Use OAuth2 authorization instead of personal tokens

[AlphaGit]

This practice is recommended by Harvest integration. However, that requires a way to recover the OAuth confirmation tokens through the user.

Consider something similar to what we have done in Google authentication (unfinished, but present), where calling the program with particular parameters will trigger a console application that will perform the operation.

http://help.getharvest.com/api-v2/authentication-api/authentication/authentication/