Optionally remove time crate dependency

[jethrogb]

Continuation of #137. I had to keep the old branch around for... reasons.

Original PR:

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I would like to use the chrono crate to process time information on systems that don't have a clock. The time crate, which is used by chrono to obtain and represent the local time, can't be made to work on such a system. This PR adds a default-enabled "local" feature to the crate and moves all code that is dependent on the time crate behind a cfg-gate. Current users of chrono need to do nothing. Users of chrono that would like to use it without the time crate can set default-features = false in their Cargo.toml.

Open questions:

• Test coverage I have fixed all unit tests such that they will pass with or without the local feature. There are however 31 doc tests that use some code that has been cfged-out. I'm not sure how to selectively disable them.
• UTC UTC::now and UTC::today also depend on the time crate. I have put them behind the same cfg-gate, but that might be a misnomer. We could leave this as is, or I could add another feature with a different name or rename the current feature.

[jethrogb]

Ah, I see we're not running with cargo test --no-default-features at all. I agree that that is unfortunate. There's no way to just not run the doctests either, afaict. I would strongly prefer to test with --no-default-features, I'll do some more research.

@quodlibetor What do you mean? I changed the way CI is run in 5e68c60#diff-b4553af39acae935d505c76e139d4aa3

[quodlibetor]

Oh I'm just confused. Yeah what you've currently got seems like as ideal as we're going to get.

[quodlibetor]

I'm really busy this week, but will release chrono 0.4.2 with this feature by the weekend.

Thanks so much for your patience!

[quodlibetor]

This is now in crates.io as chrono 0.4.2.

Thanks again for your patience!