added ability to scale background image, taken directly from augusto
Start very basic colspan / rowspan implementation
Factor out Cells#aggregate_cell_values method
Add specs around content_width available to spanned cells for drawing
Enable long-disabled spec
Colspan defaults to splitting width between all columns of the span
Ignore dummy cells on Cells#style
Re-enable final disabled spec!
Ensure row spans are split evenly between rows
Ensure cells below / right of a span are numbered correctly
Raise InvalidTableSpan when colspan / rowspan set after init
Detect and raise when spans overlap
Cells#method_missing only passes through setter methods
Fix 'dummy cells not drawn' spec -- clean up
Ensure colspan default sizing works when width is increased / reduced
Add manual page for colspan / rowspan
Ensure table row_colors restarts on every page

This ensures that when a page has an odd number of data rows, the
following page's coloring is restarted so it does not conflict with
the header color.
Add :draw_text_callback for Text::Box to intercept text drawing
This fixes the "undefined method `identifier' for nil:NilClass" problem for me.
Allow importing a stream for page template