Implement predicate validators suggested by dpom

[logaan]

• number: Should perhaps be numeric looking string.
• nil-value: Should perhaps be called present. And the current present validator should be renamed to empty string.
• greater-value: Maybe just greater than.
• less-value: Maybe just less than.
• between-value
• equal-or-greater-value
• num-greater
• num-less
• num-between

Submitted versions can be found here.