Refactoring

The refactoring promised in #5.  

There are now three separate checkers for eastwood, kibit and typed, which are applied one after another through Flycheck's standard chaining mechanism.  To disable specific checkers, just add them to `flycheck-disabled-checkers`.

The library does not implicitly register its syntax checkers anymore, to make `require` safe.  Instead you now need to explicitly call `flycheck-clojure-setup`.  Installation instructions and commentary are updated accordingly.

The error parsing is a little intricate, since typed apparently emits file names relative to the source directory of the parent project, which isn't exactly easy to resolve from the Emacs Lisp side.  I presume this can be fixed on the clojure side of this project, but I don't know enough Clojure to do that.

The new code uses the `flycheck-clojure` prefix, since I dimly remember that someone somewhere suggested that this project be renamed to `flycheck-clojure`.  If that's not the case, just tell me and I'll change the prefix to `squiggly-clojure` again.

I'll be away the next couple of days, and won't be able to respond, so please be patient.