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.
Merge pull request #205 from Atilla/proper_json

Switching JSON encoding to UTF-8.