BUG: HDFStore bug when appending to a table, .typ not recreated on subsequent appends

not correctctly recreating string columns so that could test the min_itemsize if its passed on subsequent appends, see:

http://stackoverflow.com/questions/15488809/how-to-trouble-shoot-hdfstore-exception-cannot-find-the-correct-atom-type?noredirect=1#comment22032050_15488809