_ReaderBuffer derives from _Reader
_WriterBuffer derives from _Writer
refactored _WriterBuffer._put()
added tests for _ReaderBuffer.filter() and _WriterBuffer.filter()
filtering fix for _WriterBuffer
added information to tutorial about buffers and filtering
improved buffer documentation
tweaked test_buffer.py
tweaked filter.py and test_filter.py
minor tweaks
v0.1.5