datastore: add support for omitempty in Property

Datastore does not currently provide a way to manually define omitempty in the Property struct.

The following example will not store 'Bar' in Datastore if the value = "".

``` Go
type Foo struct {
Bar string `datastore:"bar,omitempty"`
}
```
However, when defining a custom Save() method for Datastore's PropertyLoadSaver interface, there is no provided way to mimic this behavior.
I see that 'NoIndex' is a flag that provides support for the `datastore:",noindex"` tag.
If a similar flag, such as 'OmitEmpty' was provided, that would be very helpful.