aria2: Deploy aria2 download manager

The *aria2* role installs the *aria2* download manager and sets it up to
run as a system service with RPC enabled. It also sets up the web UI,
though that must be installed manually from an archive, for now.

aria2.yml

@@ -0,0 +1,9 @@
+- hosts: aria2
+  roles:
+  - apache
+  - aria2
+  tasks:
+  - name: ensure apache is running
+    service:
+      name=httpd
+      state=started