4.2.0 - validation lib improvements

• Removed the useless 'strict' length validators: removed min_len_strict and max_len_strict in validate entry point, and removed strict argument in validation_lib's minlen and maxlen. Indeed length is an integer by python framework definition, so it is always more compact to do +1 or -1 to the number. Fixes #29.

• New empty and non_empty built-in validators in validation_lib. New empty argument in validate. Fixes #31.

See documentation page for details.