Improve HAML support

[alsonkemp]

The current HAML support is minimal. Enhance to meet the specs on http://haml.hamptoncatlin.com/.

[softmechanics]

Three issues I've found:

• can't use literals for element attribute values
• can't create empty elements, i.e.
  
• can't include VHtml (i.e. output from functions, including HAML functions) in HAML blocks

For something like this: %div %br %input{type="text",value=value}

We currently have to write: %div %br=[] %input{type=text,value=value}=[] where text="text"