removeLink: Add test expecation for "removeLink" with issue #20.
removeFormat: Add test expecation regarding to issue #21.
removeFormat: Re-factor start and ending boundary points adjustment.
externs: Fix JS type annoation for |EditingContext.prototype.createElement|.
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