Support parsing from an already open file handle.

Created by: andrewyatz

This changes the code in TextParser to check if the given filename was a file handle. If it is a file handle the code just assigns it and assumes you know what you are doing.

There was also some cleaning up of the open_content() method to delegate to open() once it had created the in-memory filehandle. It seemed odd not to do once open() could support open filehandles.