Fixed sed escaping and Makefile.

The ampersand letter wasn't escaped before sent to sed, it had some weird side effects when latex expression contained ampersand, and actually produced wrong equations.

Also, Makefile didn't contain SHELL variable, so on some platforms (on my ubuntu), shell expansion didn't work.
see: http://stackoverflow.com/questions/6681624/how-to-handle-shell-expansions-in-gnu-make-under-ubuntu

It might be true that changing wget line to three separate lines would be better as no dependency on bash would be made.