ability to overload settings per instantiation

[gdibble]

For example, in my app I want to use the default of { retryCount:3 } in most cases,
but this particular fetch I would like to try more or less, i.e.

myModel1.fetch({
  retryCount: 99
});
// ||
myModel2.fetch({
  retryCount: 0
});

Any setting, default or via .set, should allow overloading (thus superseding settings) from the current instantiation.