Don't allow insecure, predictable random numbers for IVs

Silently use MCRYPT_DEV_URANDOM when
the chosen random number generator
is not one of those that are considered secure.
default $_rand to MCRYPT_DEV_URANDOM

and document that we're silently revert to MCRYPT_DEV_URANDOM
when using MCRYPT_RAND
Merge pull request #565 from kohana/3.3/bugfix/silently-replace-mcrypt_rand-with-mcrypt_dev_urandom

Don't allow insecure, predictable random numbers for IVs