Tidy representation when truncating dfs

This PR closes #5603

Dataframes are now truncated centrally (similar to pd.Series). 

Before: 
![trunc_before](https://cloud.githubusercontent.com/assets/832380/2991707/9050ffe4-dc82-11e3-8a2c-083b09dae672.png)
After: 
![trunc_after](https://cloud.githubusercontent.com/assets/832380/3000645/649bf09e-dd26-11e3-889e-33a052b13554.png)

Ipython notebook: 

Simple Index: 
![notebook_trunc](https://cloud.githubusercontent.com/assets/832380/2991894/0031dd44-dc86-11e3-8d1c-8dc8086edac4.png)

MultiLevel Index: 
![notebook_multi](https://cloud.githubusercontent.com/assets/832380/2991895/05975656-dc86-11e3-886a-c811f4fc3fe5.png)