Support for $data assertions

Usage of new assertions :

//dom
<div id="test" data-options='{"name":"John"}'  data-foo=”bar” />

- expect($el).to.have.$data(“foo”,”bar”);
-
expect($el).to.have.$data(“options”).and.to.have.property(“name”,”John”)
Merge pull request #13 from niki4810/master

Support for $data assertions