Add support for standalone true, false and null types

[IonBazan]

Signed-off-by: Ion Bazan ion.bazan@gmail.com

Q	A
Documentation	no
Bugfix	no
BC Break	no
New Feature	yes
RFC	no
QA	no

Description

Provides support for true, false and null types.

The false and null ale no longer required to be a part of a union, therefore they can used on its own. The requiresUnionWithStandaloneType() method has been replaced with a simple return false as it is used in assertCanBeAStandaloneType() and assertCanIntersectWith() - it may be replaced in the future if needed.

See: #142

[Ocramius]

Thanks @IonBazan!