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.
BUG: fix broken BigqueryError reference
Merge pull request #6104 from jreback/pytables_3.1

COMPAT: provide compat tests and better error messages for multiple file opening on PyTables >= 3.1 (GH6047)
API: Series.str wont' raise and now returns None (GH6106)
TST: add a test for columns with nested strings
Merge pull request #6108 from jreback/str

API: Series.str wont' raise and now returns None (GH6106)
ENH: ExcelWriter() accepts date and datetime formats

with ExcelWriter(date_format='YYYY-MM-DD', datetime_format='YYYY-MM-DD
HH:MM:SS') you can set the formatstrings for Excel export

BUG: xlsxwriter was not able to write date and datetime objects
Merge pull request #6107 from cpcloud/query-test-for-stringified-elements

TST: add a test for nested strings
CLN: remove docstrings from a handful of tests for consistency and nose -v output
BLD: travis, run nose with -v for more visibility
Merge pull request #6110 from y-p/PR_test_cleanups

CLN: Eliminate handful of test docstrings, use nose -v on travis
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