API: add whitelist for methods that make sense GroupBy objects
Merge pull request #4887 from cpcloud/groupby-blacklist

API: disable to_csv and friends on GroupBy objects
ENH: evaluate datetime ops in python space with eval
TST: test nondate vs. date comparisons in query
TST: add test for multiindex resolver naming
ENH: convert all datetime-like objects to Timestamp
CLN: move resolvers getter to generic.py