Add Bitmapped 2D histogram

Issues:
May need to flip the image array to get the correct histogram..
Bitmap should be made larger to prevent smoothing.
save_as_pdf does not work, working directory, relative paths etc..
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.
Change scaling from 50xbins to at least 1000 pixels
Refactored saving bitmap files
Implement assets for MultiPlot
Add MultiPlot demo

For bitmapped 2d histograms
Fix demo TeX
Do not concern with implementation

MultiPlot should not need to know that we need to save bitmaps, and
possibly other types of assets. Just call save_assets.
We change CWD, no need for build flag

We need to change CWD because LaTeX cannot find the input files
otherwise. There is a -output-directory flag, but no easy
-input-directory flag.
Updated docstrings
Use less bins in the Histogram 2D demo

These plots took about a minute to make,
more because the pdf is built twice on the first demo run..
Which is too long, this halves (~225/400) that time, acceptable or still to slow?
PEP8 fix and change line flow
Merge pull request #1 from 153957/hist2d-assets

Merge refactoring of Histogram 2D assets and support for MultiPlot
Merge pull request #10 from 153957/hist2d

Add Bitmapped 2D histogram