Show optional scrypt parameters when entering credentials

Created by: jytou

While sending a transaction, you have:

Please enter your Scrypt Salt (Secret identifier): ***************
Please enter your Scrypt password (masked): **************
Please enter your Scrypt parameters (N,r,p): default [4096,16,1]: 

The third entry should not be mandatory, since there is no reason to change these values, and it could be quite confusing to users. So I think we should remove this line, but add a switch on the command line to bring it back: for simple users, we will never bother them with it, and for those who really want to have it (I still wonder why they would), they can still have it.