Add logos to HTML versions.

	* assignments.img/assignments.png, tiger.img/tiger.png: New files.
	* tiger.css: Rename file as...
	* tiger-body.css: ...this.
	Keep only the body background property, and move the rest of this
	file...
	* common.css: ...here (new file).
	* configure.ac (tiger.css, assignments.css): Use them to configure
	these files.
	* assignments-body: New file.	
	* Makefile.am (dist_assignments_img_DATA): Add
	assignments.img/assignments.png.
	(tiger_imgdir, dist_tiger_img_DATA): New.
	(AM_MAKEHTMLFLAGS): Move --css-include option...
	(%.html: %.texi): ...here, so that every HTML file can have its
	own style sheet.