Modifications to the tests in 'pylearn2/tests/rbm/test_ais.py' in order to reduce their running time.

Running the command written below previously took longer than 4 hours, so I modified some parameters in order to reduce the running time to 20 minutes.
```
THEANO_FLAGS=mode=DEBUG_MODE nosetests test_ais.py
```
Hmm, in truth, what we need is a `beta` argument to `ais_data` (that is passed along to `rbm_ais`), and a `fast` argument to test_ais, with a default of `True`. If `fast` is `True`, use your new `beta` to get a fast-running unit test. If `fast=False` then use the defaults.

Then add a block

```python
if __name__ == "__main__":
    test_ais(fast=False)
```
So we have a way of invoking the slow test if we want to.

The truth is that you really do need to run the slow test to verify the implementation is correct. Running the fast test only tells us that the implementation _runs_.