Specs and doc for OpenGraph array of hashes support

* Added specs for opengraph image array
* Added documentation about opengraph image array

:hand: hope to be useful
Use the new Ruby 1.9 hash syntax.<br>Use 2 spaces for indentation in a hash, relative to the first position after the preceding left parenthesis.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.