HTML reprs for large dataframes.

As discussed in #4886, the HTML representation of DataFrames currently starts off as a table, but switches to the condensed info view if the table exceeds a certain size (by default, more than 60 rows or 20 columns). I've seen this confusing users, who think that they suddenly have a completely different kind of object, and don't understand why.

With these changes, the HTML repr always displays the table, but truncates it when it exceeds a certain size. It reuses the same options, `display.max_rows` and `display.max_columns`.

Before:

![pandas_long_repr_before](https://f.cloud.github.com/assets/327925/1575857/ae103a8c-5153-11e3-8a2c-f770cc76bdc2.png)
![pandas_wide_repr_before](https://f.cloud.github.com/assets/327925/1575858/ae40b22a-5153-11e3-8c1e-3d9a632a5136.png)

After:

![pandas_long_repr_after](https://f.cloud.github.com/assets/327925/1575860/b35a2caa-5153-11e3-9887-69302703f545.png)
![pandas_wide_repr_after](https://f.cloud.github.com/assets/327925/1575861/b35c7bc2-5153-11e3-872c-6a85523f93db.png)