Add `greaterThan` and `lessThan` methods

Makes a few other methods redundant.

Resolves #29, #26, and #25.

Removed:

* `aNegativeInteger`
* `aPositiveInteger`
* `negativeInteger`
* `posativeInteger`

Added: 

* `equal`
* `greaterThan`
* `greaterThanOrEqualTo`
* `lessThan`
* `lessThanOrEqualTo`
* `sameDate`

Modified:

* `today` (no longer takes second argument, instead use `sameDate`)