added Truncate

I have added a `Truncate` function (`trunc` and `truncate` are already base named functions).  It may need to be cleaned up/renamed etc.  It basically provides a truncated display of data.frames and matrices (and recursively in lists).  It may not fit with the package though.  If it doesn't my feelings will remain in tact.

I checked and it passes CRAN checks.

TR