Add App Engine support

App Engine disallows package unsafe. As a workaround for the (unsafe)
RCU atomic pointer shenanigans we pull in order to avoid taking a lock
in the hot routing path, let's just grab the lock. Honestly, I doubt
anyone will notice anyways, especially considering the fact that App
Engine is single-threaded anyways.

Fixes #52.

web/atomic.go

@@ -1,3 +1,5 @@
+// +build !appengine
+
 package web
 
 import (

web/atomic_appengine.go

@@ -0,0 +1,14 @@
+// +build appengine
+
+package web
+
+func (rt *router) getMachine() *routeMachine {
+	rt.lock.Lock()
+	defer rt.lock.Unlock()
+	return rt.machine
+}
+
+// We always hold the lock when calling setMachine.
+func (rt *router) setMachine(m *routeMachine) {
+	rt.machine = m
+}