OAuth 2.0 functionality and other fixes

I've noticed that this repository hasn't been touched in a while, and that it has never conformed to the OAuth 2.0 specs despite community efforts. That's why I took all the fixes that people have so generously commited as a pull request and combined them in my own fork. Thanks goes out to the following people for writing their fixes:
- pelle/oauth-plugin#132
- pelle/oauth-plugin#125
- @pelle for writing this gem.

The refreshing of the access token works as follows; once an access token has been acquired, [the expires_in and expires_at attributes will be set](https://github.com/RubenHoms/oauth-plugin/blob/feature/access-token-refresh/lib/oauth/models/consumers/token.rb#L50-66), if the consumer does not respond with such a paramter, the standard of 3600 seconds (1 hour) will be used and the object will be saved to the datastore. There's an [after_initialize callback](https://github.com/RubenHoms/oauth-plugin/blob/feature/access-token-refresh/lib/oauth/models/consumers/services/oauth2_token.rb#L3) on the `Oauth2Token` model which triggers only if the token has expired. If it does it will initialize the OAuth2 client and call the [refresh!](https://github.com/intridea/oauth2/blob/master/lib/oauth2/access_token.rb#L79-88) method. (thank you @greggroth for the pointer to this method) It will then save the new attributes to the datastore.

I'm pretty sure that the `after_initialize` is the way to go since most expirations are set to 3600 seconds and the object will be initialized many times in between. If anyone would know of a more clean way of doing this, please say you and I will try to fix it.

Here's an example of how to add Skydrive as an OAuth 2.0 provider in the oauth_consumers.rb file:
```ruby
OAUTH_CREDENTIALS={
  skydrive: {
    key: "YourKeyHere",
    secret: "YourSecretHere",
    super_class: "Oauth2Token", # Use this or override this for OAuth 2.0 consumers
    scope: "wl.basic wl.emails wl.skydrive_update wl.offline_access",
    options: {
      site: "https://login.live.com",
      token_url: "/oauth20_token.srf",
      authorize_url: "/oauth20_authorize.srf",
      response_type: "code",
      client_id: 'YourClientIdHere',
      redirect_uri: 'http://www.yourwebsite.com/oauth_consumers/skydrive/callback2' # Required for OAuth 2.0 providers!
    }
  }
}
```
Note that the redirect_uri option is required for the callback. Your routes.rb file should look something like this to make this work:
```ruby
resources :oauth_consumers do
  get :callback, :on => :member
  get :callback2, :on => :member # Add this line for OAuth 2.0 functionality.
end
```