Add compilation of pngfilters to installation and fix related bugs

C-result of Cython included to allow compilation during install
Revert "Add code for applying alpha channel"

This reverts commit 88c5b8fc59e6c6dd27fea02244e6390f90dfa653.
Using more native bytearray insteas of array of bytes

Speedup about 10% in unittests (python 2.7, compiled filters)