Leaky ReLU

Implemented the Leaky ReLU unit described in this paper

Maas, Andrew L., Awni Y. Hannun, and Andrew Y. Ng. "Rectifier nonlinearities improve neural network acoustic models." ICML Workshop on Deep Learning for Audio, Speech, and Language Processing. 2013.

which shares similar sparse activation properties with the ReLU, but was shown easier to optimize.
probably doesn't make any difference with `-O2`, but you could factor out top_diff[i] so there's only one multiplication (in the GPU kernel as well), i.e.:

```
top_diff[i] * ((bottom_data[i] >= 0) + negative_slope * (bottom_data[i] < 0);
```