Panel attribute naming conflict if item is named 'a'
Including an item named 'a' breaks the expected attribute-retrieval behavior for Panels

```
import numpy as np
from pandas import DataFrame, Panel

df0 = DataFrame(np.zeros([3,4]))
df1 = DataFrame(np.ones([3,4]))

p = Panel({'a':df0, 'b':df1})

p.b
Out[6]: 
   0  1  2  3
0  1  1  1  1
1  1  1  1  1
2  1  1  1  1


p.a
Out[7]: 'minor_axis'
```

The expected behavior can be recovered by using anything other than 'a' itself:

```
p = Panel({'a_':df0, 'b':df1})
p.a_
Out[9]: 
   0  1  2  3
0  0  0  0  0
1  0  0  0  0
2  0  0  0  0
```

I'm guessing this is related to the fact that many Panel methods use `a` in iterating through the axes:

```
passed_axes = [kwargs.get(a) for a in self._AXIS_ORDERS]
```

Though it's not clear immediately which is the offending use, or if changing all of these is worth it for the recovery of the `a` namespace...
