Merge pull request #6005 from jreback/pytables_3.1

TST: pytables test changes to work in 3.1rc1
ENH: improved inference of timedelta64[ns] and datetime64[ns] in the prescence NaT/None/NaN
     from object dtypes (GH5689)
CLN: refactor of timedelta routines to cython for inclusion in maybe_convert_objects
BUG: apply to a Series with a timedelta (GH5458)
ENH: improved timedelta inference for non-ns dtypes
BUG: timedelta64 merge/join issues (GH5695)
DOC: release note for timedelta changes

TST: skip tests on numpy < 1.7 as needed for timedeltas

BUG: fixup .abs for timedeltas