fix to support int labels

Integer labels get converted to `numpy.int64` objects when the input is read in, and these can't be JSON-serialized.  This causes `run_experiment` to fail when trying to write JSON results files---specifically, the descriptive statistics about the training data.

I adapted this solution for a related problem:
http://stackoverflow.com/questions/11561932/why-does-json-dumpslistnp-arange5-fail-while-json-dumpsnp-arange5-tolis

Check out this craziness:
```
ipdb> json.dumps(np.float64(4.0))
'4.0'
ipdb> json.dumps(np.int64(4))
*** TypeError: 4 is not JSON serializable
ipdb> 
```
If you're using `train_file`, you shouldn't use `featuresets`.  In fact, I thought we had it set so that it would raise an exception if you tried to use both.