BUG: fix failing usage of self.assertRaisesRegexp for python 2.6
RLS: first release candidate for v0.12.0
BUG/TST: wrong parsing of microseconds with format arg (#4152)
Merge pull request #4166 from jorisvandenbossche/bug-microsecond-parsing

BUG: wrong parsing of microseconds with format arg (#4152)
DOC: minor io/whatsnew doc edits
TST: to_datetime format fixesx

CLN/TST: to_datetime cleanup