Enable HTML like label in attributes

I have added the code for HTML like labels, and a spec.

Graphviz has [HTML like labels](http://www.graphviz.org/doc/info/shapes.html#html) to mark up shapes.
HTML like labels start with `<` and end with `>` not `"`.

A test has been passed which includes my added spec.
````
[otahi@otahiair Gviz]$ bundle exec rspec -f p  
.............................................................................................................................................

Finished in 0.08499 seconds
141 examples, 0 failures
[otahi@otahiair Gviz]$ 
````

Please check it.