Made cell.save consistent with row.save in terms of requiring a callback (#105)

[tablekat]

row.save could be called without a callback, while cell.save could not be. Made them consistent with each other, as well as consistent with the example in README.md

[theoephraim]

Maybe it's better to not let row.save be called without a callback?

The example is definitely misleading because it doesn't include them, but I think it might cause more confusion to let people call them without realizing they need a callback.

What was your use case?

[tablekat]

Any time you're adding data but don't need to read or modify it later, there's no reason to wait for the async call to finish.

My use case was just generating data and then writing it all sequentially. I found myself just repeatedly calling .save(function(){}) since I never needed to know that it finished.