Make sampling with a unicode prime work correctly

[IvanVergiliev]

Running sample.py with a unicode prime fails with something in the spirit of

sample.py: error: argument --prime: invalid unicode value: [...]

Using the approach suggested in https://stackoverflow.com/a/22947334/596167 appears to fix things.

[hugovk]

@IvanVergiliev The build failed because:

./sample.py:10: 'six.text_type' imported but unused

Please remove the import. Thank you.

[IvanVergiliev]

Done. Also, apparently the old default value (u' ') was incompatible with the new type so I changed that as well.