PERF/API: alias len/size in groupby.transform
alias `len/size` to `groupby.transform` speedup table (as these are already implemented as `size`,`count`` in cython, just  a matter of transforming them

see here: http://stackoverflow.com/questions/24438273/aggregation-on-pandas-datetime-series-only-returns-as-datetime-series/24438366#24438366
(`len` is suspect for `.agg`  as well)

so 

```
g = df.groupby(...)

g.transform(len)
g.transform('size')
g.transform('len')
```

will do the same
