Allow the clients to request a server echo of their messages.

Since the server sends messages in the same order to all clients, this
allows clients to use the server message order as the canonical edit
serialization order.  This simplifies client-side OT.
Implement OT synchronization of text fields in forms.

Uses the server echo feature to define a canonical serialization of all
changes.  If a client receives a change from a peer which serializes
before the changes they have queued, the OT delta transpose functions
are used to reorder the patches, and then the local changes are
retransmitted.

We also use the OT mechanism to handle updates to the cursor position
and selection.
Merge pull request #660 from cscott/ot-merge-2

Use Operational Transform to synchronize text fields in forms.
Add TowTruck.addTracker() method.

This lets a 3rd party add a new tracker for a specific type of page
content.
Merge pull request #661 from cscott/ext-trackers-2

Add TowTruck.addTracker() method.
Use new operator and $.each to be kinder to 3rd party trackers.

Using the new operator ensures compatibility if the 3rd party tracker uses
the standard JavaScript constructor function style.

Similarly, using $.each allows the 3rd party tracker to return a standard
JavaScript array of elements (instead of the jquery array-like object).
Avoid race condition when initializing with identity integration enabled.
Use standard $.hasClass() method; don't crash if form-init element not found.
Bug fix with OT sync.

If both you and your peer's history basis=1, it doesn't necessarily mean that
you've got the same content, since one (or both) of you might have edited
the content before you started towtruck.
Update README to say more about testing (fix #662), and fix towtruck/README.md to be accurate
Merge pull request #663 from cscott/more-fixes

A set of additional small fixes.
Ensure trackers' change monitors are deregistered when TowTruck is closed.
Merge pull request #668 from cscott/close-trackers

Ensure trackers' change monitors are deregistered when TowTruck is closed