Add is_true and is_false helper methods

Natively PHP supports a lot of variable handling functions. One of those which is used a lot is the `is_null` function. It takes away the pain of having to write `$variable === null` all the time. The function f.e. is heavily used in the Laravel Framework core (Trivia: 164 times at this point :wink:).

To be honest, I don't understand why there isn't an equivalent for `true` and `false`. It would provide the same purpose as the `is_null` function. 

With this PR I'm proposing to add these two functions.