Changed 'parenthesis' setting to 'attribute_style' setting

I removed your "parenthesis" setting and changed it to an "attribute_style" setting which defaults to the HAML "Hash Style" attributes but can be set to utilize the "HTML Style" attributes. I'm not 100% sure if the verbiage makes sense so would be open to feedback from others. 

Also, I've tackled what I thought were the correct parts of the plugin but I'm not 100% sure I've got everything covered. Specifically, I don't have a complete understanding of what is supposed to happen here: https://github.com/conraddecker/emmet-vim/blob/437cd920044185b956d6ff6b8e9f4abbc1744bbd/autoload/emmet/lang/haml.vim#L35. 

When will ```type(Val) == 2```? In addition, I've searched through the code base and can't find any other reference to ```function('emmet#types#true')```

Lastly, I'm noticing a couple errors in my local environment when I try to expand certain inputs. 
This ```nav>ul>li``` expands without a problem but when I attempt to expand ```td[rowspan=2 colspan=3 title]``` I get the following errors:

![screenshot 2014-07-17 09 52 21](https://cloud.githubusercontent.com/assets/771695/3613695/206b9bee-0dba-11e4-94af-47b40e9024b0.png)

I'm pretty certain those are a result of the changes that I made primarily because it only happens when there are attributes that need to expand but I'm not 100% certain what it is that's causing the problem. Would certainly be open to some pointers to get it cleared up.

Let me know what you think. 

Thanks!