Includes changes:

• Change Clippy to default to run on Rust nightly. We can change this once Clippy is distributed with stable.
• Various changes to make tests more robust.

Of course you will need to hook up Travis and AppVeyor to the repo. I used defaults for everything except on AppVeyor I used the Visual Studio 2017 environment (instead of 2015).

These run stable/beta/nightly Rust as separate jobs. The syntax tests run in every job.

I'm not an expert on either of these systems, so let me know if you want any changes. Some notes:

• I left notifications at the default, which just send out an email. Let me know if you would prefer something else, or if you want to add my email to the default notifiers.
• I didn't add the badges to the Readme. We can add these if you want.
• We may want to consider setting up the cron/scheduling to force these to run periodically to catch problems with Rust nightly.

I'm still a little concerned about the tests being a little fragile since some of them are testing race conditions, but I've run them a fair number of times and they seem to be doing well for now. Also, it's not unusual for Rust nightly to make minor changes to the diagnostic messages which trip up the tests. Those are pretty easy to fix, though. Nightly is also just broken sometimes.