Callable fields, Morris preUnits, postUnits & ykeys, DjangoJSONEncoder

Maybe a lot of stuff in only one PR.

For my own purposes, I have added :
 - a GraphosEncoder (which inherits from DjangoJSONEncoder) to serialize Decimal fields ;
 - options for preUnits, postCommits and ykeys for Morris charts ;
 - the possibility to define callable fields as data fields for a data source.