Dashboard widget cleanup, closes #253

* admin-dashboard.php was being included twice
* added 'null' as third argument to do_meta_boxes()
* dashboard was being included in two menus, WP Settings > as well as Jigoshop >
Closes jigoshop/111. Prevent duplicate variations
Merge branch 'release/1.2' into dev
Top selling products report
fixes #248. Shipping tax can be applied even without product tax
Most sold, top earners, total customers, total orders, total sales
Merge branch 'release/1.2' of github.com:jigoshop/redhillsranch into release/1.2
Add flot pie, remove flot from jigoshop_backend.js (frees like 37kb) and only load when necessary
Add pie charts
Use twitter tooltips
update readme
Refractor jigoshop_is_admin_page() to a smaller & cleaner function
Merge branch 'release/1.2' into topic/reports
Add some default points when using a range on the graph
fix #261
Merge branch 'release/1.2' of github.com:jigoshop/redhillsranch into release/1.2
Change 'tip' qmark icon
Few image optimizations. Total saved: ~10kb

* Losslessly optimize .gifs to .png, save a few kb.
* Delete success-off.gif because it wasn't used anywhere.
* Reduce & optimize jigowatt.png to 200px x 34px (the max size we used)
Merge branch 'release/1.2' of git@github.com:jigoshop/redhillsranch.git into release/1.2
More image optimization. Saved: ~18kb, Total (+ last commit): ~28kb

* Just some lossless optimization in this commit