Refactor gradient model selection

- added unit tests for gradient model selection (exact regression and EP binary classification), compared to GPML
- libshogun classifier_gaussian_process_binary_classification example now loads data from files
- quickly fixed regression model selection graphical example, it now outputs something like:

![figure_1](https://f.cloud.github.com/assets/2270602/1188137/4d37b25c-23b9-11e3-970e-06a32398b9b8.png)

- valgrinded