Add comparison submatchers to validate_numericality_of

You can now use the following with validate_numericality_of:

* is_greater_than (corresponds to :greater_than)
* is_greater_than_or_equal_to (corresponds to :greater_than_or_equal_to)
* is_equal_to (corresponds to :equal_to)
* is_less_than (corresponds_to :less_than)
* is_less_than_or_equal_to (corresponds_to :less_than_or_equal_to)