Allow more Trello settings

[sk7]

This is how the authorization screen looks like withe current social auth.
It says "Let An unknown application use your account?" and the auth token expires after 30 days (which I thought was anoying).

I added two optional settings:

SOCIAL_AUTH_TRELLO_APP_NAME = 'Best App Ever'
SOCIAL_AUTH_TRELLO_EXPIRATION = 'never'

-->

[omab]

Thanks!