Minor code refinement

[yihe-61wu]

I really appreciate this project, offering a light-weighted python implementation of RBM. While I have to change the code a lot for my own purpose, I refine rbm.py a little bit:

1. The code duplication between run_hidden and run_visible has been removed. A similar approach to yang-zhang's pull request is used. (I wasn't aware of his code until I decided to make this pull request myself.) But I keep run_hidden and run_visible for compatibility.
2. The logistic function has been modified. As the project is dependent on numpy, which gives overflow error for numpy.exp(x) when x is larger than 709 (for float64 if I remember correctly).
3. I'm sorry that I also changed entirely the indent for my own purpose, and it also makes the code comparison harder on GitHub. But as the topic suggests I merely made some minor refinement stated in the above two points.