CLN: refactor core/index and tseries/index,period to have their formats and to_native_types be consistent
     in naming / operation
ENH: add support for Period formatting (as a native type)
TST: add period support to makeCustomDataFrame for testing purposes
BUG/TST: format multi-index nan correctly
Merge pull request #3193 from jreback/index_fmt

CLN: refactor core/index and tseries/index,period to have their format, to_native_types methods consistent