Update hrr.py

1. extend HRR methods, such as correlate
2. compare method convolve and convolve2
   ifft(fft(self.v)*fft(other.v)).real is faster than [sum([x[k]*y[(i-k+self.number)%self.number] for k in xrange(self.number)]) for i in xrange(self.number)] 
3. compare method correlate and correlate2
   ifft(ifft(self.v)*fft(other.v)).real*10 is faster than [sum([x[k]*y[(i+k)%self.number] for k in xrange(self.number)]) for i in xrange(self.number)]