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.

roles/aria2/handlers/main.yml

@@ -0,0 +1,12 @@
+- name: reload systemd
+  command: systemctl daemon-reload
+- name: restart aria2
+  service:
+    name=aria2
+    state=restarted
+- name: reload httpd
+  service:
+    name=httpd
+    state=reloaded
+- name: save firewalld configuration
+  command: firewalld --runtime-to-permanent