Make HyList add returns HyList

Signed-off-by: Julien Danjou <julien@danjou.info>
Add a `when' macro

Signed-off-by: Julien Danjou <julien@danjou.info>
Add a `unless' macro

Signed-off-by: Julien Danjou <julien@danjou.info>
Update indentation for `if'

Signed-off-by: Julien Danjou <julien@danjou.info>
Rename `decorate-with' to `with-decorator'

Fixes #158

Signed-off-by: Julien Danjou <julien@danjou.info>