Firestore Timestamp Support

[mpfaff]

Firestore will be disabling the use of native date & time implementations with Timestamp fields in favour of their Timestamp class by (I think) the end of 2019. It is already possible in all official Firestore packages to opt-in to using the Timestamp class, but Timestamp support will soon be required to handle Timestamp fields in all Firestore packages. The JavaScript package itself already includes support for the Timestamp class, so I wouldn't expect it to be too difficult to implement in this package.