Added unfilter, decomp, buffered read methods

pngr: Added method for unfiltering scanlines. Added method for creating
scanlines (uses unfilter). Added method for buffered read of binary
chunk. Added wrappers for buffered read (for whole chunk and data only).
Made add_chunk append appropriate chunk subtype. Updated notes.

test: Removed decomp, unfilter, and buff read methods (now in core
module).