Support the optional key parameter for equalTo and endAt

[cartant]

Since I made this comment and submitted this PR, the Firebase SDK documentation appears to have been updated and equalTo and endAt are documented as supporting the optional key parameter:

This argument is only allowed if ordering by child, value, or priority.

AngularFire2 should also support the optional key parameter for equalTo and endAt.

There is a plunk here that demonstrates the SDK usage.