update documentation
turn off debug mode
extract method
hide scroll bar while taking snapshots
run partial snapshot only after scrolling is done
remove scroll paddding mechanism

fixed element will be hidden while taking partial snapshot.
they are recovered after full snapshot is done
organize actions before/after native snapshot

canvas will be built after snapshot is fully taken (not on-the-fly)
allow inaccuracy while scrolling
add dev logging method
use single shared canvas to keep full snapshot rather than maintain
actual canvas while in the process.
CRITICAL. reimplement native snapshot.

- use shared canvas as a container
- check invalid snapshot (caused by chrome API) and regenerate it
- draw on canvas after image data is loaded
update documentation
bump version