Force usage of SSL in redirect uris

Since OAuth2 does not implement any encryption itself and relies on SSL for transport security, it is wise to use HTTPS in communication instead of HTTP. Unencrypted communication allows intercepted data to be easily read. And furthermore an access token or other credentials to be easily misused. This pull request adds an option force_secured_redirect_uri to the configuration and forces the usage of https in redirect uris. This is not a complete pull request, but leaves room for (code) improvements and discussion such as:

* should https be required as default in redirect uris?
* any better name for `force_secured_redirect_uri`? 
* other protocols other than https?
Agreed on making it default `true`, I'll add parameter to allow setting it to false. 
I am not very fond of changing behaviour of methods inside code depended on the environment you are running. This can lead to confusion ("it worked on development and test but not in production") and incorrect working acceptance specs. But it should be also considered that the developer doesn't have to start a webserver in SSL mode for fast testing. I'm suggesting that developers add `force_secured_redirect_uri false if Rails.env.development?` to their initializers if they want the behaviour disabled in development. This way they can also test with SSL in development mode. What do you think?