Move json encoding out of riak_object

Revisiting https://github.com/basho/riak_kv/pull/88
But using riak_object_json encode/decode and leaving old methods in
riak_object.erl with a deprecation warning.
Addressing review comments

Using constants for links and last modified headers.
Removing old @spec.
String detection should not allow 256 int values.
Merge pull request #601 from basho/eas-dejson-riak-object

Move json encoding out of riak_object