LZWDecode filter

I needed LZWDecode filter and I found solution on [StackOverflow](http://stackoverflow.com/questions/6053064/python-pypdf-adobe-pdf-ocr-error-unsupported-filter-lzwdecode): [improved filters.py](http://vaitls.com/treas/pdf/pyPdf/filters.py).

This is just copy & paste & pull. For me this code works fine.