BUG: DatetimeIndex with time as key
DOC: specify return type in to_datetime
BUG: Option context applies on __enter__

Option context no longer overrides options when used outside a `with`
statement.

Added test TestConfig.test_option_config_scope

Closes #8514
Merge pull request #8921 from jorisvandenbossche/docstring-to_datetime

DOC: specify return type in to_datetime
Merge pull request #8925 from hkleynhans/fix/8514_option_context_with

BUG: Option context applies on __enter__
BUG: fix doctests in pandas.core.common

$ nosetests pandas/core/common.py --with-doc -v
Merge pull request #8907 from behzadnouri/time-slice

BUG: DatetimeIndex with time object as key
Merge pull request #8931 from rupertthompson/fix/core_common_doctests

BUG: fix doctests in pandas.core.common
TST: 32-bit construction fix re GH8907