Added Duns.bool(). Closes #33.

[silfverstrom]

Duns.bool() validates either exactly true or exactly false values. Duns.bool extends Duns.any and has two additional methods.

• mustBeTrue - forces value to be === true.
• mustBeFalse - forces value to be === false.

Example,

    var schema = Duns.bool();
    schema.validate(true); // true
    schema.validate(false); // true

    var schema = Duns.bool().mustBeTrue();
    schema.validate(true); // true
    schema.validate(false); // false

    var schema = Duns.bool().mustBeFalse();
    schema.validate(false); // true
    schema.validate(true); // false