Added missing 'get' for MonadState instances.

Adding a `get` function to the `MonadState` typeclass. `get` is available on `State` but not when using `StateT`.

I'm not sure how handy this function would be as an addition, but I see that it is on the haskell `MonadState` type class. This does also make the intention a little clearer during usage, as calling `init` might seem a little strange if the state already exists.

Signed-off-by: Gary Pamparà <gpampara@gmail.com>