Add more options to lenght validator

[ivanxuu]

PROBLEM: Current options for length are not enough. Maybe you need to
restrict the minimum lenght or be a exact value

SOLUTION: add all the options of rails length validator, not just
maximum. But keep the old behaviour for retrocompatybility if just a
number is passed to length.

  Examples:
    multilang :title, :length => 100  #define maximum length validator
    multilang :title, :length => {:maximum=>100}  #define maximum length validator
    multilang :title, :length => {:minimum=> 2,:maximum=>100}  #define min/max length validator
    multilang :title, :length => {:within=>2..100}  #define min/max length validator
    multilang :title, :length => {:is=> 10} #length is exactly 10