Fixed several tiny (non-mathematical) typos

This pull request corrects several tiny typos. They are all non-mathematical, so probably do not need an entry in the errata list.

I added a few non-breaking spaces, only in places where it looked really necessary to do so in my particular printing (A4).

Merry Christmas!
\url does (using a typewriter font, which can draw more attention than wanted in some contexts), but \href just displays its second argument without any special processing.

For my taste, the horizontal spacing between the slashes is too big without a manual correction. Of course, this is a matter of opinion.

There are also fancier methods of tackling this problem, see for instance http://www.joachim-breitner.de/blog/519-Nicer_URL_formatting_in_LaTeX.