remove one_hot, first try

@memimo There is no need to use spaces to call `pylearn2.format.target_format.convert_to_one_hot`.

My proposal would be:
- move `k_hot` to `pylearn2/format/target_format.py` and make it use `one_hot_formatter` with `mode='merge'`.
- move `compressed_one_hot` as well, and make it call `one_hot_formatter` instead of `one_hot`, so that it continues working.
- remove `pylearn2/utils/one_hot.py`.