ENH: speed up multi-key groupby

Improves multi-key `groupby` speed; On master:

    In [3]: pd.__version__
    Out[3]: '0.14.1-276-g995f91c'

    In [4]: np.random.seed(2718281)

    In [5]: n = 20000

    In [6]: df = pd.DataFrame(np.random.randint(1, n, (n, 3)),
       ...:         columns=['jim', 'joe', 'jolie'])

    In [7]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
    1 loops, best of 3: 1.05 s per loop
with this patch:

    In [10]: %timeit df.groupby(['jim', 'joe'])['jolie'].transform('max')
    10 loops, best of 3: 120 ms per loop