Fixes for bitmap Histogram2D

Move bitmap related stuff to SubPlot instead of BasePlotContainer
Add chdir to _build_document to make relative paths work.
Add _build_dir keyword to save_as_pdf, careful with this one,
it is deleted upon completion!
Enlarge the bitmap image 50 times (still small in KB),
prevents excessive smoothing in PDF viewers.
Flip bitmap image for proper orientation.
Update Histogram2D demo to show off bitmap vs vector.