use prototypes

[sheppard]

For example, overwriting ds.save requires a hack:

define(['wq/store'], function(ds) {

var _dsSave = ds.save;
ds.save = function(data, id, callback) {
    // custom code ...
    _dsSave(data, id, callback);
}

});

With prototypes, this could become:

define(['wq/store'], function(ds) {

ds.save = function(data, id, callback) {
    // custom code ...
    ds.prototype.save(data, id, callback);
}

});

See #20.

[sheppard]

This is superseded by #105 and the potential to use class instead.