An XOR example

Hi, 

I have created an XOR example. This could be useful for debugging. 

I had to revert the last of your commits a45b20857bc82943cbe3a2078f84948bc1438b5f, because it broke the code.

Suprisingly, it still does not learn the XOR function. 

In Pybrain, I managed to get it learn in 20 iterations.

All the best, 

Filip