ENH/VIS: Dataframe bar plot can now handle align keyword properly

Closes #4525.

I modified the `BarPlot` internal alignment to meet line coordinates. Previously, `BarPlot` doesn't pass `align` keyword to matplotlib (thus matplotlib uses `align='edge'`), but it adjusts bar locations to looks like `align='center'`.

Now `BarPlot` pass `align=center` to matplotlib by default and ticks are being locates on integer value starts from 0 (0.0, 1.0 ...). Drawing bar and line on the same axes looks like below.

*Output using current master:*
![align_incorrect](https://f.cloud.github.com/assets/1696302/2492234/0df22b26-b21c-11e3-8a96-95cffa20a7c6.png)

*Output after fix*
![align_correct](https://f.cloud.github.com/assets/1696302/2492238/5f5bc26a-b21c-11e3-9b70-76892cea9eca.png)