Enable use of index level in joins by name
```
In [35]: A
Out[35]:
       Aattr3 Aattr1       Aattr2
Aidx1
37       Delta   East         hard
59     Epsilon   East         hard
96        Zeta   East         hard
21       Gamma   West  medium hard
2        Alpha   West         soft
10        Beta   West         soft

In [36]: B
Out[36]:
              Bname  Battr
Bidx1 Bidx2
37    20579     Alan  45.20
59    20693    Betty   4.40
     20706  Charlie   9.95
37    20742   Dougal  16.60
96    20861     Emma   7.40
37    21167   Freddy  20.60
59    20947   Gareth  21.00
96    20536    Helen   9.65
37    21002     Ivan  43.80
21    21491    Jenny   6.90
     20655    Keith  42.40
2     21083  Leonard  23.60
     21391    Mandy  30.40
     21381   Nicola  11.60
10    21416   Oswald  30.30
2     21392    Peter  44.40
     21386  Quentin   5.40

In [15]: A.index.intersection(B.index)
Out[15]: Index([], dtype=object)

In [9]: pandas.merge(A, B, left_index=True, right_on=['Bidx1'])
...
KeyError: 'no item named Bidx1'

In [10]: pandas.merge(A, B, left_index=True, right_on=['Bidx1',
'Bidx2'])
...
/usr/local/lib/python2.7/dist-packages/pandas-0.7.2.dev_be66c27-py2.7-
linux-x86_64.egg/pandas/tools/merge.pyc in
_validate_specification(self)
   278             n = len(self.right_on)
   279             if self.left_index:
--> 280                 assert(len(self.right_on) ==
self.left.index.nlevels)
   281                 self.left_on = [None] * n
   282         assert(len(self.right_on) == len(self.left_on))

AssertionError:

In [18]: pandas.merge(A, B)
...
/usr/local/lib/python2.7/dist-packages/pandas-0.7.2.dev_be66c27-py2.7-
linux-x86_64.egg/pandas/core/groupby.pyc in
get_group_index(label_list, shape)
  1494         return label_list[0]
  1495
-> 1496     n = len(label_list[0])
  1497     group_index = np.zeros(n, dtype=np.int64)
  1498     mask = np.zeros(n, dtype=bool)

IndexError: list index out of range
```
