[OAuth 2] Start an authorized request by suppling an automatic mechanism of token renewal

[fabiomassimo]

Summary

As described in OAuth 2 documentation, the access token has an expiration date. The duration of an access token, together with a refresh token, is provided when the user retrieves a new access token.

This PR aims to:

• properly store the information about the expiration of a retrieved access token
• expose, in the OAuth 2 client, a convenience mechanism to automatically renew an expired token by using the refresh token.
• improve error handling by using a convenience enum struct to store all error codes

Why?

I'm working on a library that helps developers to easily access Google services. I found in OAuthSwift a perfect fit for implementing the OAuth 2 process but I wanted to extend its capabilities with a solid mechanism of access token automatic renewal only based on the access token expiration.

How would I use it?

To kick off a request that needs to be authorized simply call, on a OAuth 2 client, the following method:

oauthClient = OAuth2Swift(consumerKey: consumerKey, consumerSecret: consumerSecret, authorizeUrl: AuthenticationConstants.AuthorizeUrl.rawValue, accessTokenUrl: AuthenticationConstants.AccessTokensUrl.rawValue, responseType: AuthenticationConstants.ResponseType.rawValue)

oauthClient.startAuthorizedRequest(urlString, method: .GET, parameters: parameters, headers: headers, onTokenRenewal: { (credential) in
           // Convenience fallback to store new credential somewhere safe (i.e. the keychain)
            do {
                try self.credential.storeOAuthCredential(credential)
            } catch {
                failure(error: error as NSError)
            }
        }, success: success, failure: failure)

[fabiomassimo]

Wow! Thank you so much!

[phimage]

thx, good work

[antwerpenR]

See my comment on #217. I think there may be a bug - at least when used with Meetup.com.