refactor(scope): watch "readOnly" arg renamed to "canChangeModel"

[vicb]

closes #739

This changes is a BC break:

• the "readOnly" parameter has been renamed to "canChangeModel",
• the semantic is inversed

Before:

// Default to digest (readOnly = false by default)
scope.watch(exp, fn);
// Explicit flush
scope.watch(exp, fn, readOnly: true);

After:

// Default to digest (canChangeModel = true by default)
scope.watch(exp, fn);
// Explicit flush
scope.watch(exp, fn, canChangeModel: false);