ENH: Added more options for formats.style.bar
 - [x]  4 tests added  and passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] Update documentation

You can now have the df.style.bar be centered on zero or midpoint value (in
addition to the already existing way of having the min value at the left side of the cell)

----

## Documentation:

I have produced the following example that I think does a pretty good job of explaining the new capabilities

```
import pandas as pd
from IPython.display import HTML

# Case 1: s.min() < 0:
test1 = pd.Series([-100,-60,-30,-20], name='All Negative')
# Case 2: s.max() < 0:
test2 = pd.Series([10,20,50,100], name='All Positive')
# Case 3:
test3 = pd.Series([-10,-5,0,90], name='Both Pos and Neg')

head = """
<table>
    <thead>
        <th>Align</th>
        <th>All Negative</th>
        <th>All Positive</th>
        <th>Both Neg and Pos</th>
    </thead>
    </tbody>

"""

aligns = ['left','zero','mid']
for align in aligns:
    row = "<tr><th>{}</th>".format(align)
    for serie in [test1,test2,test3]:
        s = serie.copy()
        s.name=''
        row += "<td>{}</td>".format(s.to_frame().style.bar(align=align, 
                                                           color=['#d65f5f', '#5fba7d'], 
                                                           width=100).render()) #testn['width']
    row += '</tr>'
    head += row
    
head+= """
</tbody>
</table>"""
        

HTML(head)
```

![screen shot 2016-11-28 at 17 06 21](https://cloud.githubusercontent.com/assets/5479063/20675547/04c4af52-b58d-11e6-9695-48a01c37d522.png)

What do you think?



