round of pylint fixes.
add experimental, maybe hack(ish?) Context Manger to manage frames.

The purpose is to streamline and unclutter the allocation/deallocation
in presence of errors and exceptions.
export new image method.
fix planes attribute; sketch out bytes(). WIP.
let decode() always return a list.

KISS. having a different return value type is not handy: is nasty.
sketch and refactor single plane access. Still (VERY) WIP.
add two more examples. Still very rough and undocumented.
Pull from HG.