Fix handling of timestamps in BigQuery insert data method

[dhermes]

Fixes #1247.

[dhermes]

I just realized Bigtable has me trained too much. Wrote Bigquery instead of BigQuery in the commit messages.

[dhermes]

FWIW I cherry-picked the second commit onto head and ran the system test with the error reported by @kamatama41 and got the following:

======================================================================
ERROR: test_load_table_then_dump_table (bigquery.TestBigQuery)
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Traceback (most recent call last):
  File ".../gcloud-python/system_tests/bigquery.py", line 232, in test_load_table_then_dump_table
    rows, _, _ = table.fetch_data()
  File ".../gcloud-python/.tox/system-tests/local/lib/python2.7/site-packages/gcloud/bigquery/table.py", line 611, in fetch_data
    rows_data = _rows_from_json(response.get('rows', ()), self._schema)
  File ".../gcloud-python/.tox/system-tests/local/lib/python2.7/site-packages/gcloud/bigquery/_helpers.py", line 81, in _rows_from_json
    row_data.append(converter(cell['v'], field))
  File ".../gcloud-python/.tox/system-tests/local/lib/python2.7/site-packages/gcloud/bigquery/_helpers.py", line 42, in _datetime_from_json
    return _datetime_from_microseconds(1e6 * float(value))
  File ".../gcloud-python/.tox/system-tests/local/lib/python2.7/site-packages/gcloud/_helpers.py", line 235, in _datetime_from_microseconds
    return _EPOCH + datetime.timedelta(microseconds=value)
OverflowError: date value out of range

----------------------------------------------------------------------
Ran 11 tests in 69.796s

[tseaver]

LGTM. OK to merge w/ pylint failing on Travis, pending #1248.

[dhermes]

I'll just wait for #1248

[dhermes]

@tseaver Assuming your LGTM still holds after rebasing.

[kamatama41]

Great job! Thank you for fixing quickly 😄

[dhermes]

Sure thing. Thanks for reporting and finding the bug.