Added `or_else` and `get_or_else`

`Maybe.from` is based off of your README.md

`or_else` is aliased from `mplus` like it is in Haskell. See http://www.haskell.org/haskellwiki/Monadplus

I'm not sure if there is an aliased `getOrElse` in Haskell, I'm taking this from scala http://www.scala-lang.org/api/current/index.html#scala.Option. This allows one to get back to non-Maybe world where things expect `nil` and `null` or whatever else evaluates to boolean `false` in Ruby land.

I've never seen a line of Ruby before today, so if there is some idiomatic Ruby I fudged, let me know. I needed these for something I'm working on.