Add feature flag for time 0.3 support

[FalkWoldmann]

Hi,
I've seen that you provide a feature flag with support for chrono 0.4. Would it be possible to add an additional flag for time 0.3? It looks like bson already supports this as a feature flag.

[felschr]

Just to clarify, mongodb 2.6 can already be used with time by adding bson to ones dependencies with the time-0_3 feature flag.

The bson-{x}-style feature flags of mongodb just affect the reexported module.
Of course it's desirable that the corresponding flag for time will be added eventually.

[isabelatkinson]

Hi @FalkWoldmann, thanks for filing this issue! While we do currently have some feature flags in the driver exposing bson features, this is a practice we have decided to move away from, and we plan to remove the bson-related feature flags in our next major release. Instead, we suggest users do what @felschr outlined above: add bson as its own dependency in your Cargo.toml file and toggle whichever feature flags are desired there.