Add support for additional object types

[mburtscher]

Add support for additional object types:

https://developers.facebook.com/docs/reference/opengraph

[BrettGregson]

I'm keen to help you with this

[mburtscher]

I'd appreciate your help!

As you can see on the Facebook docs page (linked in issue description) there are multiple object types defined for the OpenGraph protocol. Each of those should have it's own implementation in the Fusonic\OpenGraph\Objects namespace and derive from ObjectBase class.

The Consumer class decides which object should be instantiated. Additional object types must be added here:

https://github.com/fusonic/opengraph/blob/master/src/Fusonic/OpenGraph/Consumer.php#L108