Move some csvn classes to more logical places. Add comment to top of csvn/types.py to clarify what belongs there.
* csvn/core/__init__.py, csvn/types.py (Hash, _stream_read, _stream_write, _stream_close, Stream, SvnStringPtr): Move these classes from core/__init__.py to types.py.
* csvn/types.py, csvn/repos.py (LogEntry): Move from types.py back to repos.py, since the LogEntry class is not a generic Subversion type, but is in fact directly related to Subversion repositories. (_LogMessageReceiver): Move to bottom of file to make module easier to read.