TST fix broken tests (which were using fancy asserts, broke travis py2.6 and py3)
Merge pull request #3200 from hayd/fancy_asserts

TST fix broken tests (which were using fancy asserts, broke travis py2.6...
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