Bar plot fails if an axis parameter is supplied
Example code:

```
from pandas import DataFrame
import matplotlib.pyplot as plt

df = DataFrame({'a': [0, 1], 'b': [1, 0]})

fig = plt.figure()
ax = fig.add_subplot(111)
df.plot(ax=ax, kind='bar')
plt.show()

UnboundLocalError: local variable 'axes' referenced before assignment
```

If the 'ax' parameter is removed, no problem. Line plots with a custom 'ax' work too.
