make all text node changes use REPLACE_TEXT

until now some text node changes would be caught by REPLACE_INNER_HTML and others by REPLACE_TEXT. This should make sure all pure text node changes are caught by REPLACE_TEXT.
find inner attribute differences

As explained in issue #53, attributes of inner nodes are not (always) found. This change takes care of the test case.
rename REPLACE_TEXT to MODIFY_TEXT_ELEMENT and add undo for it
merged all changes