Wildcard for indexing with MultiIndex
based on [mailing list discussion](https://groups.google.com/forum/#!searchin/pystatsmodels/wildcard/pystatsmodels/vxUE3NXWpVM/sYnBKkYZOLgJ), adding here for better traceability.

Is there a way to use wild cards when using MultiIndex?

As an example, let`s work with the following DataFrame:

``` python
In [10]: df
Out[10]:
         0        1       2       3       4
x  y  z -0.02688  1.236  -1.174  -1.026   1.508
a  b  c  0.9156   1.742   0.4483  1.005   0.3641
p  q  r  0.2019  -0.3895 -0.8484  0.9992 -0.2274
```

Selecting all rows where level 0 of the MultiIndex == 'a' is easy:

``` python
In [11]: df.ix['a']
Out[11]:
      0       1      2       3      4
b  c  0.9156  1.742  0.4483  1.005  0.3641
```

But what if i want to select all rows where level 3 of the MultiIndex == 'c', and i don`t care what the other levels are?
This is what can be done:

``` python
In [12]: df.swaplevel(0, 2).swaplevel(1, 2).ix['c']
Out[12]:
      0       1      2       3      4
a  b  0.9156  1.742  0.4483  1.005  0.3641
```

Wes idea is that this merits a new API function or adding to an existing one.
e.g. to modify DataFrame.xs to work like:

``` python
df.xs('c', axis=0, level=2)
```

Other idea is to do something something similar as in numpy, and use : as wildcard.
For example in numpy:

``` python
In [7]: y = np.arange(27).reshape(3,3,3)

In [8]: y

Out[8]: array([[[ 0,  1,  2],
        [ 3,  4,  5],
        [ 6,  7,  8]],

       [[ 9, 10, 11],
        [12, 13, 14],
        [15, 16, 17]],

       [[18, 19, 20],
        [21, 22, 23],
        [24, 25, 26]]])

In [9]: y[:, :, 0]

Out[9]: array([[ 0,  3,  6],
       [ 9, 12, 15],
       [18, 21, 24]])
```

y[:, :, 0] indexes all elements with "level 2" == 0.

This would translate to df.ix[:, :, 'c'] on the first example.
Another example on a DataFrames with MultIndex with a lot of levels and selecting on more than one level: df.ix[:, :, 'c', :, 10]
This approach looks familiar, and no other api method is needed for indexing when using wildcard.
