The YouTube API seems to want the URL to start with https://.

It gives cross-window origin mismatch errors unless the URL comes from
https://www.youtube.com.
Replace old towtruck slogan
Clarify tracker vs trackerName.

Try to make it clear in the code when we are passing around the *name* of
a tracker (a string) and when we have an actual tracker object.
Clarify element vs elementPath.

Try to make it clear in the code when we are passing around a jquery
wrapper of the actual DOM element, and when we are passing around
a path specification for that element (a string).
Add `setHistory`/`getHistory`/`getValue` helpers for trackers.

The `setHistory`/`getHistory` methods allow trackers to define their own
methods to store/set history for a tracker.  The `getValue` helper is
extended to call `tracker.getContent` where appropriate.
Pass `sendData` to trackers; allow `maybeSendUpdate` override.

Allow trackers to define their own implementation of `maybeSendUpdate`
(in particular, if they would like to use a different format for their
update messages).  Pass the `sendData` method to the tracker constructor,
so that trackers outside of togetherjs core can use the same techniques
which the built-in trackers use.
Factor out `{make,serialize,parse}Delta` functions.

This allows trackers to use their own representation of "changes" while
sharing the basic `ot.SimpleHistory` framework.
Refactor tracker init/makeInit methods to reuse common code.

Trackers no longer need to define `init` and `makeInit` methods if they
define a `getContent` method and can reuse the standard ot mechanism.
Custom trackers may define a `makeInit` method to override the built-in
history mechanism.  They can also define `parseInitValue` and
`serializeInitValue` methods if their `getContent` method returns
a complex object which is not immediately serializable.
Better handle race conditions during form initialization.

Add a server-generated timestamp to the `init-connection` message, and use
this to identify the "oldest session" when arbitrating between multiple
received `form-init` messages.  Also defer updates between the time
that `form-init` is sent and when it is received echoed back from the
hub, in order to ensure that the newly-inited peer does not start up
with stale state.
Multiple hellos from the same client shouldn't confuse form init.

Improve the form-initialization algorithm introduced in
eb6519ea9cb144b01cec51933dd6fb6d0380f1a1 by adding a unique identifier
to the `hello` message to ensure we don't get confused if we get
multiple `hello` messages from the same client in quick succession
(as we might, for example, if the user double-clicked "reload" in their
browser).
Ensure that init-connection is received before tests start.
Add sauce labs testing support

Sauce Labs doesn't seem to support websockets over sauce connect yet,
so we can only do the unit tests, not the functional tests.  Still,
it provides some testable boundaries for browser support.