Simple Syntax, Correct Accept Header Responses

In this release, we've cleaned up the location declaration language, thanks to @timjb's IsString generic instance and type-level refactoring of the type accumulator. We've also eliminated the subtle Either in each request, so each handler can be made with a call to handle, instead of the old handleLit and handleParse. Also, we have Accept header discrimination - if the header declares that it can't handle text/plain, then we eliminate Text from the lookup.