Fix refcount error and use cythons <bytes> to convert a python object to char*
Support unicode objects for paths

Use sys.getfilesystemencoding() for encoding
Separate strictly between bytes/unicode

which makes py3 support possible
Fix import to support python3
Add a text about bytes and unicode in pyrocksdb
Use bytes for the internal representation of a WriteBatch
Update docu about the bytes and unicode
Tag all the C++ functions with nogil and except+