ENH/VIS: Dataframe bar plot can now accept width and pos keywords 

Following keywords are now acceptable for `dataframe.plot(kind='bar')` and `dataframe.plot(kind='barh')`.
`width`: Specify the bar width. In previous versions, static value 0.5 was passed to matplotlib and it cannot be overwritten.
`pos`: Specify relative alignments for bar plot layout. From 0 (left/bottom-end) to 1(right/top-end). Default is 0.5 (center).

*Examples:*
![figure_1](https://f.cloud.github.com/assets/1696302/2428540/7496a148-ac4c-11e3-9964-8c3353551606.png)