Fixed scrypt N-param bug and added time calc

Fixed an error in the way the N parameter was being using in SCrypt, should be using 2^N instead of just N.
Also added benchmarking to the scrypt calculation so we can start to get a handle on scrypt execution time for different tunings of the parameters
Added export master key example
Tweaking scrypt params to increase compute time

Standard on-device compute times should be around 2 seconds
Exporting master key time should be around 60 seconds