Nicer readmes using markdown.

This is still readable in raw text, but much nicer for github with heading and syntax highlighting and such.

Also this includes some nice badges that shows the build and coverity status.

NOTE! Please go to https://ci.appveyor.com/project/warmcat/libwebsockets/settings/badges and copy the proper Markdown URL for Appveyor, it uses some random URL for each repo (I couldn't guess yours).