Create array of enums

Creating an array of enum doesn't appear to be implemented, would be nice to have!

```
var mySchema = new Schema({
  label: [{type:String, enum: ['enumVal1','enumVal2']}]
});
```

Small snippet of conversation on irc which might give direction:

```
aheckmann: file an issue, i think its b/c the validation isn't set on the array
```
