Modify black scholes example to use internal RNG from library instead of std::rand

[roshanrags]

This adresses #276 . Also check if the values are correct... (esp. the put price)
On my setup,

$ ./example/black_scholes
device: GeForce GT 650M
option 0 call price: 15.189
option 0 put price: 0

UPDATE: Travis also reports the same value, so I guess the problem of different values on different platforms is solved. I have no clue about correctness though...

[kylelutz]

The black-scholes code should still be correct, but different input values will give different output values. Having 0 exactly for the put price is a little weird, I'd try changing the RNG seed value to something else and then use those results.