Improve class hierarchy of package org.apache.wicket.util.crypt

[Andrea Del Bene]

Hi,

Following the work done for WICKET-5756, I thought it would be nice to 
have a more flexible and clear hierarchy for cryptography classes in 
Wicket 7. I opened WICKET-5768 and I reworked those classes in a 
separate branch called WICKET-5768-improve-crypt. The main purpose of my 
work is to remove method ICrypt#setKey(final String key) because this 
assumes we want to use a string as "secret" key.
Such assumption makes (imho) the adoption of other cipher (AES, 3DES, 
etc...) not so natural/intuitive, which of course doesn't mean 
impossible: see inner class 
UnlimitedStrengthJurisdictionPolicyTest.UnlimitedStrenghtJurisdictionPolicyCrypt. 

My changes imply API breaks, but I think it's worthwhile to take this 
opportunity and clean the inner API before we freeze master branch for 
milestones.

What do you think?