Change default RPC mechanism to websockets

[EliAndrewC]

We currently use jsonrpc as the default RPC mechanism, but we should make websockets the default. This is especially important in cases where a Javascript client wants to subscribe to something and shouldn't need to know or care on what box the plugin actually lives.

When we do this, we should also update the Subscription class to no longer need a URL but instead merely specify a method.

[EliAndrewC]

After spending a little time on this, it seems like it would be easiest to make a bunch of changes all at once, because they're all interconnected, so I'd want to make the changes already listed, as well as the following:

• upgrade ws4py to the latest version, because that will support websocket client certs (see https://github.com/Lawouach/WebSocket-for-Python/pull/98)
• change rpc_services to allow per-destination client certs
• update WebSocket class to accept and use client cert options

[robdennis]

I like this approach

[EliAndrewC]

A few other changes:

• we no longer need a SubscriptionJsonrpc class, since that was kind of a holdover from the days when we had one central plugin-like server that everyone wanted to connect to; since that's no longer true, that class is deprecated
• jsonrpc in general will be accessible through the services api, e.g. you can either say services.foo.bar() to make the call over a websocket connection or say services.jsonrpc.foo.bar() to make the same call over jsonrpc
• rpc services can be defined as jsonrpc-only to support defining services which aren't running on Sideboard plugins (although they'd still need to be using rpctools or something morally equivalent)
• sideboard.threads was moved to sideboard.lib.threads and refactored to have better integration with the other classes
• sideboard.lib.WebSocket objects previously connected immediately when instantiated; now that's the default behavior, with an optional argument that indicates the WebSocket connection should instead be made when sideboard starts (used for objects instantiated while sideboard is starting)
• even after all of this, the websocket server needs to be smarter to account for the new websocket-based API; if someone subscribes to a service call that's actually a remote service, we need to turn that into a subscribe rather than a call; not a big deal now that the underlying plumbing is in place, but definitely some extra code

I've implemented most of this and updated the documentation for the most part, so I'll probably make a pull request tonight.

[EliAndrewC]

I'm not ready for a pull request yet, but if someone wants a sneak peek at the refactor, here's a link to the commit in my forked repo: https://github.com/EliAndrewC/sideboard/commit/4ab9318a7d6bd3ccd603fa3ed1bc660e20dc4aa4

Overall very little will change, except that there are a few more optional methods and attributes and keyword arguments to functions. The main API improvement is to the Subscription class, which is way simpler now.