Preparation: Moved color code into one method

The method setupContext is now used to set up the drawing context for a
particular frame, setting its fill- and strokeStyle.
Implements #89.
Rearranged preload indicator legend and added entry

Text is still missing because I don't know the font (in fact, as I'm currently on a Linux machine, it's quite possible I don't even *have* the exact font).
Replace the legend png with an equivalent SVG file.

This closes #89.
Removes color declaration duplicity

ayop-preload.js now fetches the colors from preloadindicator_legend.svg.
Remove debug code

whoops
Fix formatting

A few indentations in index.html were still tabs
Merge pull request #93 from lucaswerkmeister/implement-89

Implement #89