implemented document.adoptNode, fixed potential issue with node.removeChild

lib/document.js
+ implemented 'adoptNode' method

test/document.js
+ added tests for 'adoptNode' and its side effects

lib/node.js
* fixed potential issue with comparing the DOM instances, which may have been cleaned up from the underlying 'Simple' instance. Switched to verify the 'Simple' instances instead.

lib/simple/simple.js
* added 'instance' property as a default member (initialized as null)