Introduce $(VARNAME) as a ${VARNAME} alternative.

UPDATED DESCRIPTION

With this patch ${VARNAME} is resolved only after parsing the YAML 
file such that it only works with !obj, !pkl and !import tokens, as 
well as objects that perform an explicit call to string_utils.preprocess. 
All objects in databases.* do it for file paths, as well as pylearn2.Train 
for save_path. some_obj.yaml_src will thus contain the *un*substituted
version of the YAML source.

The old preprocessing behaviour is availabe using $(...).

This fixes https://github.com/lisa-lab/pylearn2/issues/300