Added info about append to docstring. Append now checks to make sure they're both strings.
Added tests for ia --append ALSO FIXED A BUG WHICH MADE IA --MODIFY NOT WORK
(said but was the metadata being in a variable called "md", but used as "metadata".)
(py.test is skipping the actually testing the modify metadata stuff, because
of not being able to tell it has permissions or something.)