Better handling of unicode characters in sample names

[isaacovercast]

Writing pandas dataframes to file buffers fails with unicode in sample names. Each assembly step will complete, but then writing stats files will fail with:

"Encountered an unexpected error (see ./ipyrad_log.txt)
Error message is below -------------------------------
writelines() argument must be a sequence of strings"

To reproduce:
Edit any of the simulated barcodes files and swap one of the letters for ö. Run steps.

Useful/related:
pandas-dev/pandas#680
https://stackoverflow.com/questions/38786936/pandas-convert-unicode-strings-to-string

[isaacovercast]

Use io.open and you can set the encoding to utf-8. This was useful: https://stackoverflow.com/questions/6048085/writing-unicode-text-to-a-text-file

[isaacovercast]

Fixed in v.0.7.24