Organized and expanded manual

I started working on putting all the documentation into markdown files in the repo, so the docs can be versioned withe app.  I discovered /docs was used for the phpDocumenter auto-generated docs, so I decided to call these pages the manual.  I broke out the sections of the README, and then added in pages from the Wiki.  Also added a StackOverflow answer I wrote, and a Gist someone else wrote.

In manual/Routing.md, I added some stuff about overriding the behavior where multiple routes are matched.  As you mentioned in the Issues section, it's a useful, but confusing behavior, so I figured it deserved some attention.  However, I'm a little fuzzy about the merits of Klein::skipRemaining vs. Response::send(), so I'd love some input on that.

Hope someone finds this helpful!