added primes example in Octave (Matlab)

Added a primes example in Octave.
It is very slow compared to Python.

jeri@hudson:~/shootout/primes/octave$ time ./primes.m
...
real    0m0.755s
user    0m0.748s
sys     0m0.004s

jeri@hudson:~/shootout/primes/python$ time ./primes.py
...
real    0m0.029s
user    0m0.020s
sys     0m0.008s