Markdown: Replace Discount Markdown library with peg-markdown

Remove Discount due to stupid licensing issues. Apparently Discount's
style of BSD license cannot be linked into a GPL software.

Now using the much simpler but also much slower peg-markdown library.

TODO: update Waf build system
Markdown: Allow choosing between Discount and peg-markdown at compile time
Markdown: Fix Autotools to support bundled peg-markdown

Default is to try and use system's Discount library and if not
available or configured with --enable-peg-markdown, use the
bundled peg-markdown library.
Markdown: Fix Waf to check/link to external Discount library

It makes no attempt to use the built-in peg-markdown library or add
the configure option like Autotools has to enable using it. I have no
idea how to do this in Waf.
Merge pull request #89 from codebrainz/markdown-depends

Markdown: Remove embedded Discount lib and other stuff

As it's really a serious issue I'm going to merge the PR und ask everyone for testing.