Define brand image in CSS instead of an <img> tag to allow it to be styled; add high resolution tentacle brand image for retina/high-res displays; create non-clashing light version of tentacle brand image to be used when darker custom colors are chosen in settings
Merge branch 'master' into responsive

Conflicts:
	octoprint/static/css/octoprint.less
	octoprint/templates/index.jinja2