Update to Maintain Compatibility
If you're storing a byte array -- i.e., *not* UTF8-encoded text -- you should be using a `BlobField`. @coleifer Thank you for your help! I've almost got it working with 'Blobfield'. 
This change allows compatibility between projects formerly using peewee2.6.2.

When trying to use the latest version of peewee we get the following errors relating to `_StringField`: "Could not get key 'utf-8' codec can't decode byte 0xb4 in position 3" or similar.

Making this change allows easy compatibility.

See issue #262 in neo-python project here: https://github.com/CityOfZion/neo-python/issues/262