Associate a debug_info field with unnamed column references
Pass along field name when converting to an unnamed ref
Rename exception for accessing a non-grouped term in an aggregate
UnnamedAttributeRef: restore $ for position
Merge pull request #266 from uwescience/preserve_attribute_name

Pass along better debug info when accessing a non-grouped field in an aggregate expression
Merge branch 'master' into proper_repr

Conflicts:
	raco/expression/expression.py