Allow passing a single object to Model.create.

[dxg]

Addresses #159

Enables:

Model.create({name: 'jane'}, function(err, item) {
  // item is not an array, but a single object
});
Model.create([{name: 'jane'}], function(err, items) {
  // items is an array
});

It seems that you can pass the same parameters to Model.create in many different ways.
Eg:

Model.create([{name: 'jane'}, {name: 'john'}], function (err, items) { ... } );
Model.create([{name: 'jane'}], [{name: 'john'}], function (err, items) { ... } );
Model.create([{name: 'jane'}], function (err, items) { ... }, [{name: 'john'}] );
Model.create([{name: 'jane'}], function (err, items) { ... }, {someOption: false}, [{name: 'john'}] );

are all equivalent.

This PR limits this somewhat, as the first argument which is an object but not an array is assumed to be a single item to be created.

I think this is resonable.
I also think the way parameters are accepted is a little too flexible. Requiring that the first parameter is the item(s) to be created, and last parameter is the callback (with an optional middle options param) would make the code a bit easier to read.

[vendethiel]

I don't understand how you space

[dxg]

haha.. normally i would write: if (!single && Array.isArray(arguments[i])) { but there was a lot of brackets happening towards the end so I spaced it out for readability. I don't really mind it either way though.

[fjenett]

+1

I think less options of how to pass arguments are better to remember and will on the long run help the project as you can read other people's code, share and so on ...

[dxg]

Will revisit the way arguments are passed once @dresende is back

[dresende]

This is ok for now, maybe we can improve later if someone is not confortable with it.