Add Password checking related validation rules

[jwillp]

Here are some levels used by applications to determine the strength of a password:

1. Very weak: any character is allowed (example: "1234567890" or "qwerty")
2. Weak: at least one lower and one upper
3. Medium: at least one lower, one upper and one number
4. Strong: at least one lower, one upper, one number and one special character
5. Very strong: at least one lower, one upper, one number, one special character and a minimal length of 12

In order to support these various requirements, the following rules would be needed:

// Where numberCharacters means the number of required characters to match  
// strict = true means at most the provided number 
// while strict = false means at least the number of characters provided
new ContainsUppercaseCharacters($numberCharacters = 1, $strict = false, $message = null); 
new ContainsLowercaseCharacters($numberCharacters = 1, $strict = false, $message = null);
new ContainsNumericCharacters($numberCharacters = 1, $strict = false, $message = null);
// For Special characters, to allow users of the library to define their own set of special chars
new MatchesRegex($regex, $message);

// Min Length
new MinLength($length, $message);
new Length($length, $message); // For Exact length