Write functions

Added the reverse operation numpy array -> IDX format. 

Some questions:
1. Do you have a better way to deal with the bug http://bugs.python.org/issue2263 which affects python 2.6 than what I did starting on line 177 in converters.py?
2. What is a good way to write unit tests for convert_to_file? Creating new files as part of a unit tests seems strange, but maybe I'm being too precious.