Added hashes and arrays as possible meta tag content

Fixed some errors on the documentation and added nesting and arrays functionality:
```ruby
set_meta_tags :foo => {
  :bar => "lorem",
  :baz => {
    :qux => ["lorem", "ipsum"]
  }
}
# <meta property="foo:bar" content="lorem"/>
# <meta property="foo:baz:qux" content="lorem"/>
# <meta property="foo:baz:qux" content="ipsum"/>
```