COMPAT: fix numpy 1.9-dev deprecation warnings in test suite
COMPAT: change neg of boolean to inv (deprecation in numpy 1.9)
COMPAT: remove deprecation warnings on **new** (in computation/pytables)
COMPAT: fix array_to_datetime on invalid comparison to NaT

related #6958

add ability to set an environmental variable to actually show the DeprecationWarnings

just

`setenv PANDAS_TESTING_MODE 'numpy_deprecate'`

for an example https://travis-ci.org/jreback/pandas/jobs/23758981
