Fix datetime encoding

This CL fixes encoding of time.Time elements, which was broken in the
following three ways:

  - datetime zone was not coerced to UTC.

  - datetime elements were not printed together with the rest of simple
    types (because isStructOrMap returned true for time.Time), and were not
    separated by a newline.

  - arrays of datetime elements resulted in a panic (because isTOMLTableType
    did not handle them as a primitive type).

---
It would be perhaps good to add tests to [BurntSushi/toml-test](https://github.com/BurntSushi/toml-test) that catch all of these.
In the meantime, I added a couple tests to TestEncode.