nodes: Add test cases
nodes: Modify comments
editing: Make |EditingContext.prototype.removeAttribute(element, name)| not to throw an exception when attempting to remove non-existing attribute.
editing: Make |EditingContext.prototype.splitNode| to use |EditingContext.prototype.removeAttribute| instead of |Element.prototype.removeAttribute|.
Merge pull request #22 from hajimehoshi/nodes-add-test-cases

nodes: Add test cases
nodes: Add tests
Merge with master