BUG: Fix to_datetime to properly deal with tz offsets #3944

Currently for certain formats of datetime strings, the tz offset will
just be ignored.
DOC: Clarify that DataFrame string formatters must return strings

I am not 100% sure that string formatters must really return *unicode* strings
(non-unicode strings seem to work, too), but I am leaving that because it's
what is already documentated.

Fixes #6052.
BUG: fix broken BigqueryError reference
TST: add tests for old version of numexpr
BLD/CI: add numexpr==1.4.2 to build wheels
CLN/TST: DRY skip_if_no_ne
Replace network decorator with with_connectivity_check

Network decorator was ignoring test failures due to API changes in
external services.
Merge pull request #6109 from cpcloud/ne-version

TST: add tests for old version of numexpr
BLD: enable Machine field in print_versions
BLD: enable Machine field in print_versions
BLD: travis, run nose with -v. again.
TST: HTMLFormatter does not die on unicode frame GH6098
BUG: HTMLFormatter does not die on unicode frame GH6098
Merge pull request #6112 from y-p/PR_GH6098

BUG: HTMLFormatter does not die on unicode frame GH6098
TST: get_options_data ValueError if given only one of month/year GH6105
BUG: get_options_data should raise ValueError if given only one of month/year GH6105
Merge pull request #6114 from y-p/PR_GH6105

BUG: get_options_data should validate it's arguments GH6105
TST: add _skip_if_no_xlrd() to test GH5583
Merge pull request #6115 from y-p/PR_GH5583_fix_skip

TST: add _skip_if_no_xlrd() to test GH5583
TST: Add xlsm to Excel parsing tests.

Added an Excel xlsm file to the excel.py reader tests.
Test for issue #4172