hy/compiler.py: When unquoting forms, call _wrap_value() on them first

Since we do not want to let non-hy values enter the Hy AST if at all
possible, when unquoting things, call _wrap_value() on them before
returning the form. This fixes #469.

Based on the ideas of Nicolas Dandrimont <nicolas.dandrimont@crans.org>
and Paul Tagliamonte <tag@pault.ag>.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>