Add getOrRethrow into feither

[geirolz]

Hi guys! This PR adds one useful method to re-throw an exception wrapped as Left into an F where a MonadThrow is available.

I'm not sure about the naming, I'd like to continue using "raise" instead of "throw" but I named it "rethrow" because it's also present into EitherT. Other alternatives that came to mind are:

• getOrRaise ( overload of the existent method )
• getOrReRaise

Example

val result: Try[Int] = Try(new RuntimeException("BOOM!").asLeft[Int]).getOrRethrow