Refactoring

I did some minor refactoring in preparation to add an eraser tool (which I haven't actually done yet.)

Changes made:

Added htdocs/Page.js and htdocs/PencilTool.js

htdocs/script.js: 

- Changed so that it no longer uses global variables inkUsage, empty, pages, pageInkUsage

- Now uses 4 PictoSwap.Page objects to store it all

- global variable page is now an array of PictoSwap.Page objects 

- renamed page to currentPage

(How this works:
What was in page[i] is now in pages[i].data.
What was in pageInkUsage[i] is now in pages[i].inkUsage.
What was in empty is now in pages[i].empty. )

- Encapsulated the code inside of canvas.onmousemove/up/down into a new PencilTool object

- Canvas.onmousemove/up/down now calls currentTool.onCanvasMousemove/up/down (currentTool is currently just pencilTool but in the future can be changed)

Everything else should still be the same.
Indentation is also weird here, probably because of mixed tabs and spaces.