exposed prettyModule, corrected example and spelling

[lue-bird]

Elm.Pretty.prettyModule is now exposed (#36):

{-| Pretty prints a module definition.
-}
prettyModule : Module -> Doc

Apart from that, some spelling mistakes and the prepareLayout example were corrected and unused imports were removed.

[rupertlssmith]

Thanks, a helpful set of changes and corrections to my terrible typos.

Can publish this as a minor version bump along with #37 (infixExpose).