ENH: Excel output in non-ascii encodings

ENH/TST: Support for non-ascii encodings in DataFrame.to_excel

Closes #3710.

Notice: Despite (for my modest knowledge of python) it should be easier to just put encoding='ascii' in the declaration of to_excel in line 1352 of frame.py, I've declared it with encoding=None as jreback suggested, and then making it default to ascii later with the if clause.