Add main/err routes to app; add template content.

Add:
	app.py
		-add route for main page, "index.html", and two handlers
		for 404 and 500 errors that load error/(404, 500).html
		respectively.

	templates/layout.html
		-add favicon link and changed {{title}} to "block title"; instead of
		manually passing in a value from app.py, we'll inherit a block
		from child templates.

	templates/errors
		-to contain all error pages.