utf8 encoding issue with cypher

[NicolasDenoyelle]

Hello,
I ran into encoding issue using encrypt/decrypt in bittrex.py, because json.dumps does not support raw encoding of cypher.encrypt.
The "encoding" parameter of json.dumps/json.loads still make python complaining when using latin1 instead of utf8.
I found an alternative solution using base64 encoding/decoding. Can you please update bittrex.py with the changes to fix the issue.
bittrex.zip