Handle various character encodings better.

[sanjuroj]

Currently, if a utf-8 encoded csv file is uploaded, it will save to the database and display fine. However, if you try to edit a cell, a Unicode error gets thrown because it can't encode the utf-8 characters with the ascii codec, which is the default in Python 2.

It would be good to handle this better, perhaps using utf-8 as the default, rather than ascii.