Add the ability to define datatypes in the configuration file.

Datatypes are defined like:
[galaxy:datatypes]
bed = galaxy.datatypes.interval:Bed
png = galaxy.datatypes.images:Image,image/png

where the mime-type (default of text/plain) can be declared after the class name, as is seen with png.

tools/data_source/encode_import_code.py

@@ -117,7 +117,7 @@ def exec_after_process(app, inp_data, out_data, param_dict, tool, stdout, stderr
             data.name = data.name + " (" + description + ")"
             data.dbkey = dbkey
             data.info = data.name
-            datatypes.change_datatype( data, file_type )
+            data = app.datatypes_registry.change_datatype( data, file_type )
             data.init_meta()
             data.set_peek()
             app.model.flush()