Expand `else` to `True` instead of `otherwise`.

Makes `if` syntax independent of the value of `otherwise`.
Includes test case.
Optimize True branch in multi-way if code generation.
Merge pull request #412 from maxsnew/otherwise

Expand `else` to `True` instead of `otherwise`.
Cosmetic: give function a more descriptive name
Add Maybe.map; clean up docs
Add extract : a -> Maybe a -> a
Add keepJusts, for extracting from signals.
Add justIf. Makes a Maybe based on a predicate.
Remove import of otherwise

Now that evancz/Elm#412 has been fixed.