No documentation for datetime bindings

[pganssle]

As part of #200, adding proper documentation to the Python datetime bindings was deferred to a later PR. This issue is to track the progress of adding proper rustdocs to the bindings.

To Do:

• src/ffi/datetime.rs
• src/objects/datetime.rs

The ffi bindings hew fairly closely to the datetime C API, so in general that can serve as a basis for the ffi documentation.

If someone else is interested in dipping their toes in the water, I'm happy to let someone else do the PR.