Callable fields, Morris preUnits, postUnits & ykeys, DjangoJSONEncoder

[gpichot]

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.

[akshar-raaj]

@gpichot Can you please check why the travis build is failing.