BUG: sort_index/sortlevel fails  MultiIndex after columns are added.
I have a `DataFrame` with a `MultiIndex` on the columns.  The first level of the MultiIndex contains `str`ings.  The second, `float`s (though the problem persists if the second level is `int`s).  I add a column to the `DataFrame` (which should _not_ come last if the columns are sorted).  I try to sort the `DataFrame`.  The result does not seem to be sorted.  The behavior is fine if the columns are simply an `Index` (even after adding columns).  And the sort works fine in the `MultiIndex` case as long as no columns have been added since the `DataFrame` was created.

MWE:

```
import pandas as pd
import numpy as np

np.random.seed(0)
data = np.random.randn(3,4)

df_multi_float = pd.DataFrame(data, index=list('def'), columns=pd.MultiIndex.from_tuples([('red', i) for i in [1., 3., 2., 5.]]))

print df_multi_float

#OUTPUT
        red                              
          1         3         2         5
d  1.764052  0.400157  0.978738  2.240893
e  1.867558 -0.977278  0.950088 -0.151357
f -0.103219  0.410599  0.144044  1.454274
```

This sorts just fine as it isnow:

```
print df_multi_float.sort_index(axis=1)

#OUTPUT
        red                              
          1         2         3         5
d  1.764052  0.978738  0.400157  2.240893
e  1.867558  0.950088 -0.977278 -0.151357
f -0.103219  0.144044  0.410599  1.454274
```

But if I add columns to both this `DataFrame and then show it sorted, I get what looks to be a wrong result (the new column remains last, rather than being placed second-to-last as it should be):

```
df_multi_float[('red', 4.0)] = 'world'

print df_multi_float.sort_index(axis=1)

#OUTPUT
        red                                  red
          1         2         3         5      4
d  1.764052  0.978738  0.400157  2.240893  world
e  1.867558  0.950088 -0.977278 -0.151357  world
f -0.103219  0.144044  0.410599  1.454274  world
```

I'm able to produce this behavior on two systems.  The first runs Pandas 0.14.0 and Numpy 1.8.1 and the second runs Pandas 0.14.1 and Numpy 1.8.2.  This issue is described here: http://stackoverflow.com/questions/25287130/pandas-sort-index-fails-with-multiindex-containing-floats-as-one-level-when-col?noredirect=1#comment39408150_25287130
