BUG: bar plot can now handle bottom and left kw properly
Actually not a bug, but allows `bar` and `barh` plot to accept `bottom` and `left` kw respectively to specify the starting point.

Currently, passing `bottom` or `left` keyword results in `TypeError`

```
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randn(5, 5))
df.plot(kind='bar', bottom=1, ax=axes[0])
# TypeError: bar() got multiple values for keyword argument 'bottom'
```
### After fix

```
import matplotlib.pyplot as plt
fig, axes = plt.subplots(1, 2)

df = pd.DataFrame(np.random.randn(5, 5))
df.plot(kind='bar', bottom=1, ax=axes[0])

df2 = pd.DataFrame(np.random.rand(5, 5))
df2.plot(kind='barh', left=[1, 2, 3, 4, 5], stacked=True, ax=axes[1])
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/3074746/24f8bebe-e351-11e3-9506-ac7de60db7fd.png)

@TomAugspurger pls review when you can
 I'm fine labeling this as a bug. `**kwargs` is supposed to pass along all the args of the original matplotlib function/method. `left` and `bottom` are args for `bar`.
 Merged via https://github.com/pydata/pandas/commit/4adc6fa62895496a3fbbf36aca876ec91ca0d47b

Thanks @sinhrks 
