Unicode support in charts

Created originally on Bitbucket by mcfoton (Mikhail Nazarov)

Hey guys. Thanks for awesome work!

I've tried create a chart and got

#!

'ascii' codec can't encode characters in position 0-9: ordinal not in range(128)
Exception Location:	.../openpyxl/utils/__init__.py in quote_sheetname, line 182

The string that could not be encoded/decoded was: Количество пост

Tried to avoid it but got into same error with reference.py line 72