Merge pull request #3841 from jreback/GH3611_2

BUG: GH3611 fix again, float na_values were not stringified correctly
DOC: cooking.rst update
Merge pull request #3804 from jreback/ujson

ENH: add ujson support in pandas.io.json
TST: Fix missing import in io/tests/test_json …

Nose import is missing. If you get to the error at the last line (under
urllib), python complains because nose is never imported.
Merge pull request #3855 from jtratner/patch-1

TST: Fix missing import in io/tests/test_json
TST slicing regression test
Merge pull request #3858 from hayd/TSTassign_subset

TST slicing regression test
ENH: Add unit keyword to Timestamp and to_datetime to enable passing of

    integers or floats that are in an epoch unit of s, ms, us, ns
    (e.g. unix timestamps or epoch s, with fracional seconds allowed)
    (GH 3540)
TST: disallow slicing a timeseries with floats

TST: manage truediv in py3 for unit comparisons
BUG: make sure that nan/none like values to Timestamp are returned as NaT