Updated recipes/sly

SLY is a young project and its file structure has changed, so this
recipe must be updated.

See https://github.com/capitaomorte/sly/issues/16

I don't foresee any more big changes in the near future.
Remove README.md and NEWS.md from the recipe
Merge pull request #2021 from capitaomorte/sly-structure-update

Updated recipes/sly