load_cardio() and load_letter() do not work under Ubuntu 14.04

[yzhao062]

While running comb_example.py, the program may fail due to loadmat() function.
A quick workaround is to use synthesized data instead of real-world datasets.

This only affects comb_example.py. Will be addressed in the next release.

[yzhao062]

Added a try-catch to load the data. If failed, the synthesized data will be used so no error would be raised.