Support null type variables

[VManolas]

Do you think it is a good idea for testphase to support
{"type": "null" }
as it is described http://spacetelescope.github.io/understanding-json-schema/reference/null.html

[nohponex]

I think we should allow a schema to be nullable and not just null type, although only null might be useful

[nohponex]

phramework/validate#17