Add support for accepting UNIX timestamps

cloud_monitoring.get_metric_data_points operates on a start and end
timestamp, but previously the function was unable to directly receive a
UNIX timestamp value, only Python date/datetime or string-based
timestamps. utils.to_timestamp now returns numbers given to it, as
they're already timestamps. This change makes it work as the docstring
suggests.

Also added a comment to get_metric_data_points to explain the existing
conversion of UNIX timestamps to integers. The API currently times out
when given timestamps with fractional seconds.
Merge pull request #178 from briancurtin/working

Add support for accepting UNIX timestamps to get_metric_data_points