Add Poison.encode_to_iodata(!)/2

Having an explicit function plays nicer with dialyzer
and makes Poison simpler to use as callbacks.

Ideally we would deprecate the :iodata option in
encode and titghen its typespecs but I haven't done
so in this commit in order to keep backwards
compatibility.
Merge pull request #19 from josevalim/jv-poison

Add Poison.encode_to_iodata(!)/2