reset_index behaviour
shouldn't the last two examples remove the innermost index level?

``` python
In [69]: from pandas.util.testing import makeCustomDataframe as mkdf
    ...: mkdf(4,2,r_idx_nlevels=1).set_index("C_l0_g0",append=True,drop=False)
Out[69]: 
C0              C_l0_g0 C_l0_g1
R0      C_l0_g0                
R_l0_g0 R0C0       R0C0    R0C1
R_l0_g1 R1C0       R1C0    R1C1
R_l0_g2 R2C0       R2C0    R2C1
R_l0_g3 R3C0       R3C0    R3C1

In [70]: from pandas.util.testing import makeCustomDataframe as mkdf
    ...: mkdf(4,2,r_idx_nlevels=1).set_index("C_l0_g0",append=True,drop=False).reset_index(0)
Out[70]: 
C0            R0 C_l0_g0 C_l0_g1
C_l0_g0                         
R0C0     R_l0_g0    R0C0    R0C1
R1C0     R_l0_g1    R1C0    R1C1
R2C0     R_l0_g2    R2C0    R2C1
R3C0     R_l0_g3    R3C0    R3C1
```

``` python
In [71]: from pandas.util.testing import makeCustomDataframe as mkdf
    ...: mkdf(4,2,r_idx_nlevels=1).set_index("C_l0_g0",append=True,drop=False).reset_index(1)

/home/user1/src/pandas/pandas/core/internals.pyc in insert(self, loc, item, value)
    956     def insert(self, loc, item, value):
    957         if item in self.items:
--> 958             raise Exception('cannot insert %s, already exists' % item)
    959 
    960         new_items = self.items.insert(loc, item)

Exception: cannot insert C_l0_g0, already exists
```

``` python
In [72]: from pandas.util.testing import makeCustomDataframe as mkdf
    ...: mkdf(4,2,r_idx_nlevels=1).set_index("C_l0_g0",append=True,drop=False).reset_index(-1)

/home/user1/src/pandas/pandas/core/internals.pyc in insert(self, loc, item, value)
    956     def insert(self, loc, item, value):
    957         if item in self.items:
--> 958             raise Exception('cannot insert %s, already exists' % item)
    959 
    960         new_items = self.items.insert(loc, item)

Exception: cannot insert C_l0_g0, already exists

```
