Switching JSON encoding to UTF-8.

By default, python's JSON encoder will escape all non-ASCII
characters, using a sequence of 6 bytes, for the sake of legacy
non-UTF-8 compatible clients. This is a significant overhead in the
case of non-ASCII string and can be reduced by using a modern encoding.

I saw no side effects from this change so far and the Ruby client is already encoding things properly. Discussion about this can be found here: http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-February/010908.html