make path param parsing router agnostic

Do not use gorilla/mux explicitly to parse route params. The route is
simple, so just a split will work fine. This allows for using other
routers[1] without needing to change the proxy ServeHTTP method.

[1]: such as httprouter, pat, Goji, etc.
attempt to use httprouter
allow for gotest flags
use a custom dumb router

go-camo has simple needs, and a very simple dumb router is good enough.
This reduces dependencies, as well as keeps the actual proxy ServeHTTP
handler more generic -- and able to be plugged into larger apps if
desired.
put router in its own place