Rethink when HTML repr of DataFrame is displayed
Helping out with a moderately beginner class recently, I noticed several people having problems, because they could easily display a table view of a small DataFrame, but the representation looked completely different when it exceeded a certain size. People thought that they had a different type of object, or that the detailed information was some kind of an error message. There's no obvious way to get the HTML repr for larger DataFrames.

Suggestions:
- Increase the size limit for displaying the full HTML repr. When we did force larger dataframes to display as HTML tables, the IPython notebook can easily handle substantially larger tables than the current cutoff.
- When the DataFrame is too large to display whole, produce a truncated HTML table rather than switching to a completely different kind of repr.

I'll try to work on this soon if no-one objects or beats me to it.
