Changed id return type for init methods to instancetype
Fail when trying to use TUDelorean with a nil NSDate

Added an assert value to the initWithType:date: method from TUTimeJump
to make it fail when a nil date is passed as argument
Fail when trying to use TUDelorean with a nil NSDate

Added a parameter assert to the initWithType:date: method from TUTimeJump
to make it fail when a nil date is passed as argument
Merge pull request #2 from vcorugedo/master

Added assert to avoid setting nil date value when using TUDelorean