[ZL-874] Manually refresh and revoke tokens

[njbbaer]

Ticket: ZL-874

This PR adds functionality to allow clients to manually refresh and revoke their token.

Pretty simple:

client = Procore::Client.new( ... )
client.refresh
client.revoke # client will refresh token on its next request

[njbbaer]

Thanks for reviewing @PatKoperwas, but I still need to add the revoke functionality before I'll request review and merge it.

[njbbaer]

@PatKoperwas I finished the revoke functionality, so it's ready for review.

[njbbaer]

@PatKoperwas @gilmcquillan Thank you for the comments, I've updated it.

[gilmcquillan]

Did you want to add a comment above def revoke

# @raise [OAuthError] if a token cannot be revoked.

[njbbaer]

Thanks for the reviews. I'll merge this as soon as I'm satisfied it works with the sample app.