Avoid libuv resource leaks (fixes #395)

include/mbgl/util/uv.hpp

@@ -21,13 +21,18 @@ namespace uv {
 
 class loop {
 public:
-    inline loop() : l(uv_loop_new()) {}
-    inline ~loop() { uv_loop_delete(l); }
+    inline loop() {
+        if (uv_loop_init(&l) != 0) {
+            throw std::runtime_error("failed to initialize loop");
+        }
+    }
 
-    inline uv_loop_t *operator*() { return l; }
+    inline ~loop() { uv_loop_close(&l); }
+
+    inline uv_loop_t *operator*() { return &l; }
 
 private:
-    uv_loop_t *l;
+    uv_loop_t l;
 };
 
 class mutex {