address deprecation of calendar='gregorian' in CF v1.9 (issue #256)

[jswhit]

add a DeprecationWarning if calendar='gregorian' used, replace DatetimeGregorian with DatetimeStandard (but keep DatetimeGregorian for backwards compatibility)

[jswhit]

We could add a CFversion attribute to cftime.datetime and only raise the warnings if CFversion >= 1.9. The question will then become what should the default value be?

[jswhit]

DeprecationWarning has been removed, and cftime.datetime.__add__ and __sub__ still return DatetimeGregorian objects so as not to break xarray. The only thing this PR really does is create a DatetimeStandard class and switch calendar='gregorian' to calendar='standard' internally everywhere.