Updated Makefile for changes in man page generation.
More Makefile adjustments.
Fix links to the spec

All the links the spec were broken. I changed `http://jgm.github.io/CommonMark/spec.html` to `http://spec.commonmark.org/0.13/`. It'd be nice if there was a `http://spec.commonmark.org/latest/` so you don't need to change the README every time a new version (unless there's major changes so the URL fragments aren't valid anymore.

The spec itself has some broken fragment links, e.g. `#image` instead of `#images`. Unfortunately I don't have a good tool that checks fragments so I don't have the full list.
Merge pull request #243 from maxlinc/patch-1

Fix links to the spec
Remove stray character
Merge pull request #244 from petere/patch-1

Remove stray character
Expose failure to normalize whitespaces
Merge pull request #245 from cirosantilli/normalize-whitespace-fail

Expose failure to normalize whitespaces
Fixed whitespace normalization (#245).
Added a few more doctests for HTML normalization (#245).
Add tests for normalize outer whitespace removal

-   expose failure with ``' <p>a</p>'`` which remains unchanged,
    ``'<p>a</p>'`` expected
-   rephrase it slightly to mention that it strips inner spaces also
    as in `<p>a </p>` to `<p>a</p>`