Undesired UX behavior of DataFrame output in IPython Notebook
From my previous line comment at https://github.com/pydata/pandas/commit/be80898920eab8f740cf61019327621a75d73947#commitcomment-11338872. cc @takluyver @jorisvandenbossche

Because DataFrame's `_repr_html_` returns hardcoded `div` tag with style `max-height:1000px`, if the height of DataFrame's output exceeds 1000px, it will generate a scroll bar.

First, it is unnecessary because IPython Notebook has "toggle output scrolling" feature. Second, it causes UX problem if the height of browser resolution is below 1000px (user must scroll twice: the whole page and the generated div). Third, it can't make use of "toggle output scrolling" feature (if user turn it on, it will generate nested scroll bars (worse UX)). For more detailed explanations and examples, see the steps to reproduce below.

Pandas version: 0.16.1.

Steps to reproduce:
1. Open an IPython Notebook and paste this code. It will generate an inner scroll bar because the output's height exceeds 1000px.
   
   ``` python
   import pandas as pd
   pd.DataFrame(list(range(100)))
   ```
2. If you want to see all rows, first you should scroll the page.
   ![a](https://cloud.githubusercontent.com/assets/1947204/7887439/3ba2e788-065b-11e5-8be1-9e9cdb4eb304.png)
3. After you reach the bottom of the generated ouput's `div`, you should scroll again the inner scroll bar.
   ![b](https://cloud.githubusercontent.com/assets/1947204/7887460/705b87f0-065b-11e5-827c-a4c7d915e485.png)
4. If you want to toggle the IPython Notebook's "toggle output scrolling" feature, you get this.
   ![c](https://cloud.githubusercontent.com/assets/1947204/7887471/8620d4d2-065b-11e5-9386-8b8f08a62c07.png)

Desired behaviors:
- When "output scrolling" feature is off
  ![d](https://cloud.githubusercontent.com/assets/1947204/7887506/c8d131b4-065b-11e5-8936-9636094ed99b.png)
- When "output scrolling" feature is on
  ![e](https://cloud.githubusercontent.com/assets/1947204/7887515/d85116a4-065b-11e5-9691-7110988f95f1.png)

Solution:
Remove style property from `div` tag that DataFrame's `_repr_html_` returns.
Old: `<div style="max-height:1000px;max-width:1500px;overflow:auto;">\n`
New: `<div>\n`

Why should `max-width` be removed as well? Because it also generates _far_ worse UX. If you have so many columns that the horizontal scroll bar generated, you cannot scroll it. You must refresh the page to be able to scroll it.
