BUG: Fix DatetimeIndex.insert() with non-datetimes.

Coerces to object Index instead.
COMPAT: allow string concats to datetimeindex only (int/float are invalid)
Merge branch 'datetime-index-insert-non-datetimes' of https://github.com/jtratner/pandas into jtratner-datetime-index-insert-non-datetimes

Conflicts:
	doc/source/release.rst