* Added `save_sheet` command that saves a range into PNG sheets, complete with trim, gap, margin, columns, and sometimes automagically computed rows. See samples/saves.rb.
* Unit conversion! Now you can write "2in" and it will convert based on the current dpi. `save_pdf` not supported (yet).
* `png` now supports resizing, but warns you about it since it's non-ideal. Documented in yard, tested.