add simple interrupt handler class for optimzation process.

This pull request contains a simple class monitoring the standard SIGINT signal from
the environment. It uses the C standard header <csignal> and can be used to add an external stopping signal.

Usage:

StoppingInterrupt interrupt;
while(opt.step() && !interrupt.isSignaled()) {
    // debug output
}


Can be integrated into each optimize() method of the corresponding OpenANN::Optimizer objects.