Pull req #1104 - change config name (Breaking)

new config option is "use_message_type" - default false (Breaking)

Also return errors if the type_name interpolation fails

Currently this doesn't error in the coordinates.go, so it can result in a
situation where interpolation silently fails when use_message_type is true, or
"Type" isn't in the fields list
Merge branch 'ESjson-fields' of github.com:markabey/heka into ESjson-fields