try: allow empty body

Signed-off-by: Julien Danjou <julien@danjou.info>
try: accept a simpler form

Signed-off-by: Julien Danjou <julien@danjou.info>
Merge branch 'jd/try' of git://github.com/jd/hy into pr-82
Merge branch 'master' into pr-82