Password minimal length

I could pass the following step with a 2 char length password. I don't know if password minimal length is ever set in CuteCoin or in uCoin. But it should ask for a minimal 4 or 6 char password and refused shorter one.

Let's ask for at least 6 characters in next version. https://github.com/ucoin-io/cutecoin/commit/8f35e30300865821a82e8ce3fbdd73ad61b6b95b