ENH: allow 'size' in groupby aggregation
Allow to use `'size'` in groupby's aggregate, so you can do:

```
df.groupby(..).agg('size') 
df.groupny(..).agg(['mean', 'size'])
```

---

http://stackoverflow.com/questions/21660686/pandas-groupby-straight-forward-counting-the-number-of-elements-in-each-group-i
- ~~`count` should directly implement `size` (enh)~~
- `count`/`size` should be allowed in an aggregation list (the bug)
