Fix incompatibility with HAML Object Reference ( tested with Rails 3.2.x )

Hi,

great work with this gem, I really like it.

I noticed a bug caused by the prefix feature in HAML's Object Reference (HOR).

Since they expect prefix as a symbol, something like 
```haml
 %span[@user, :author] # where user.id is 1
```
is expected to become
```html
<span id="author_user_1" class="author_user"></span>
```


Thus the green_monkey's usecase 
```haml
%span[@user, :performer]
```
would not result in a span with the itemprop :performer

To solve the issue, I changed the override of HAML::Buffer::parse_object_ref, so that itemprop names are passed as strings prefixed with a hashtag, so to get the intend result for the performer, one has to do
```haml
%span[@user, '#performer]
```

I know that simply switching to a string is enough, but I think that # provides some context, thus increasing code readability.

Thank you again for this gem, very nice idea and super-easy to use!