ENH: Change truncation ellispsis to unicode character

Changes the truncation symbol from '...' to the math characters horizontal, vertical and diagnol ellipsis as proposed in https://github.com/pydata/pandas/pull/7086#issuecomment-42862717. 

The the Ipython Notebook, the result looks like this

http://nbviewer.ipython.org/gist/bjonen/1683e2c49d11bc238fbf

To do: 
* Do we need to fall back to simple strings in certain environments (https://github.com/pydata/pandas/pull/7086#issuecomment-42999103)
* Implement this for pd.Series as well
* Adjust tests