Added a validator for strings.

Input are generally strings (sometimes arrays), so a validator specialized in strings makes sense.
When the Validation will be adapted to handle the new format (static functions), the current BaseValidator will be removed.
Adapted Validation.php to the new StringValidator

- Validation now check for a default value when a key is required but not found in inputs.
Changed the isBool rule: removed `true` and `false` strings so that only bool, 0 and 1 are allowed.
Removed the checkRules method

No need to have a protected function that will be called from only one place.
Reducing function calls = improving speed
Removed unused constant RULE_NOT_EMPTY. The same result can be achieved with a `minlen` rule of 1.